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Final Fantasy Record Keeper is a Free to Play Mobile game developed by DeNA based off, well, all the Final Fantasy games.
It's notable for being actually good. Unlike almost every other F2P Mobile game.
Story
There's a magical kingdom and... you know what. It doesn't matter. This game technically has a plot. But beyond the tutorial it doesn't really come up. The story is essentially an excuse to recruit characters from every mainline Final Fantasy game (Plus Final Fantasy Tactics) and fight enemies and bosses from said game.
Battle System
FFRK uses the classic ATB battle system from Final Fantasies IV to XII. A characters ATB Bar fills up, they choose an action. You can choose to Attack, Defend, use one of two abilities you have equipped on a character, or use a Soul Break.
Soul Breaks are used when a character does enough actions and/or gets hit enough times to fill up a bar on their Soul Break meter. These are basically the Limit Breaks/Overdrives/Trance Attacks/Quickenings/Etc from past FF games. Generally, however, the Soul Breaks that characters naturally have aren't that good. However, there are ways to get better ones. More on that later.
Characters
There are a lot of characters in FFRK. Since its using every single Final Fantasy game as a source, you can imagine how big the playable cast is.
Each character can equip one weapon, one armor, and one accessory. Each character obviously has their own unique equipment list. Cloud, for instance, can equip Knives, Swords, and Katanas. Obviously he's never gone to use a Harp. Abilities are also based off the character you have. There's the Final Fantasy mainstays like Black Magie, White, Magic, and Summoning. But also more unique skillsets to translate mechanics from other FF games like Samurai, and Knight.
Record Materias are gotten from using a Memory Crystal on characters that have reached the level cap. This increases their level cap and lets them drop these while playing. They give unique effects that equipment doesn't or can't give. There are wayyy to many to talk about.
Dungeons
Dungeons are the core part of FFRK. There are three types of Dungeons you have to worry about. Realm Dungeons, Event Dungeons, and Daily Dungeons
Realm Dungeons are the "story mode" of FFRK, as it were. Each Realm Dungeon is generally waves of regular enemies, ending with a boss fight. These are always available and a good source of the main materials you need to play the game. Orbs, Mythril, and the only way to upgrade Stamina. Every 5 Stamina shards you earn from the Realm Dungeon your Stamina goes up by 1, thus letting you use this skinner box a little longer. Realm Dungeons have both a "Classic" version, which is the simpler version you'll play through the first time to progress, and an "Elite" Version. Which are harder, optional to progression, and provide better rewards. Some characters can be recruited from the Realm Dungeons, but most are gotten from...
Event Dungeons are limited time only dungeons. Each Event generally follows a similar pattern to a series of Realm Dungeons. Waves of regular enemies ending with a boss. However, each event also has at the end of them powerful boss only stages. These give the best rewards the game has. Furthermore, an Event is generally the main way you recruit characters. If you don't clear enough of the event to recruit everybody, don't worry, the characters will be back another time. Plus, you can always use the "Soul of the Hero" items they also give you to acquire characters that way.
The final Dungeon is the Daily Dungeon. Unlike most dungeons, which you clear once and then move on. You're going to be running the Daily Dungeons a lot if you don't have anything else to do. Why? Because the Daily Dungeons drop all a bunch of stuff constantly. Orbs, Relic Upgrade materials, money, EXP and Gyshal Greens.
Abilities
There are a lot of abilities in FFRK. There's the classic Black Magic, White Magic, and Summoning. But there's also esoteric abilities like Knight and Support.
Abilities are made using the Orbs you get from all the Dungeons. You acquire the required Orbs, select the ability you want to make, and you use it!
However, each ability can only be used so many times in a Dungeon. Most abilities you make, you can use twice before you run out and your character has to settle for "Attack." However, you can extend the number of uses your abilities have by "Honing" them. Once honed they gain more uses. However, honing takes more Orbs then it would take to make a second copy of the ability! It's up to you to decide if more orbs are worth it rather then just spread around the ability to multiple slots. (Since you only have 10 slots, this is usually the case though.)
Relics, Soul Breaks, and the Gatcha.
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All right. Now that I've explained how the game is played, it's time to go over the parts of the game that a F2P mobile game is required to have.
This game, like a lot of Japanese F2P games, uses a Gatcha system. Rather then just buy what you want, you instead buy a chance to get what you want.
Unlike a lot of Gatcha games, however, this game does not tie characters to the Gatcha. After all, I already explained how you get them. Instead, equipment is what the Gatcha gets you. You pay for a roll on the Gatcha and you have a chance to get a "Relic", which is what gear is called in this game.
Gear is ranked. From 1 star, to 8 Star. However, the Gatcha will only give you gear from 3 to 6 Star. You get 7 and 8 Star gear from combining two of the same relic. Two of the same 4 star Relic will make a 5 Star Relic, and so on. However, these "Combined Relics" will not be as strong as a relic that's naturally that ranking. But there's still a use for these, more on that later.
The ability to draw is paid for by two currencies. Mythril and Gems. Mythril is given out for free. Clearing out any Realm Dungeon and most Event Dungeons will give the you a Mythril. This is one of the main things FFRK has over most F2P Mobile games. It's incredibly generous with this free currency. There are hundreds of Realm Dungeons out now, and each Event gives 10 Mythril throughout its Event Dungeons every 8 days. You're given 1 Mythril a day, with 3 days out of 10 giving you a second one.
Gems are the currency you buy with IRL cash. The rate is simple enough, 1 US Dollar = 100 Gems. Both Mythril and Gems are used to buy the same things, and there's no difference in effect. For Gatcha pulls you can even use a mix of Mythil and Gems if you don't have enough of one or the other. (1 Mythril = 60 Gems.)
When you finally choose to make a pull, you are then given a 14% chance of given a 5 Star relic. Does this sound low? Yes, yes it is. But remember, this is actually pretty high compared to a lot of F2P Gatcha games. Plus, there's one major change recently added to the game that makes the game a lot more worthwhile. You see, you can pay for a single pull with 5 Mythril or 300 Gems (100 Gems if it's your first pull on that "Banner") You could pay for three pulls at once for 15 Mythril or 900 Gems, or you could pay for 11 pulls for the price of 10, 50 Mythril or 3000 Gems. Oh yeah, one more recently added bonus for the 11 Pull.
If you do an 11 Pull, you are guaranteed a 5 Star or better Relic.
So with a 11 Pull, it goes from a 14% chance, to one 100% chance at least. And remember, the game is incredibly generous with Mythril. Even if you do nothing but log in each day you'd be able to do around 1 Pull a month.
Relics are the source of new Soul Breaks for your characters. Some Relics offer "Shared" Soul Breaks, which can be used by any character that can equip the relic. However, most Soul Breaks are unique to a single character. They're far more powerful then the default Soul Breaks your character has, and which characters you get to use will be defined largely by what relics you pull. As you fight battles with the Soul Break equipped, it'll gain experience. Once its EXP bar is maxed out, the character has Mastered the Soul Break, and can now use it without having the Relic Equipped. Now, not all Relics are created equally naturally. Even within 5 Star relics there's been a fair amount of variation. There's your Soul Breaks, and then there's the better stuff.
First, we have the basic Soul Break Relics. Still, mostly, better then a characters default. Generally nowadays if you'll get decent damage and an effectt. These are, of course, good. And there are some Soul Breaks that even today are desired more then higher ranking counterparts.
Moving up, we have the Super Soul Break. These are a tier above regular Soul Breaks. They do more damage and generally have stronger or simply more effects. Furthermore mastering a Super Soul Break or higher SB will give the character a permanent stat boost, just to make them a little bit better.
Next there are Burst Soul Breaks. BSBs are actually more like sidegrades to a SSB. Using a BSB will use an attack or effect generally weaker then an SSB effect. However, upon using it the character will gain "Burst Mode." This grants them a substantial stat boost, as well as replacing their Attack and Defend commands with "Burst Commands;" powerful abilties that are unique to the Brust Mode.
Finally, there's the Overstrike Soul Break. This is the newest tier of power, having just come out recently. OSB Relics are the first natural 6 Star relics, and are pretty dang powerful. Using an OSB will make the character do an absurdly powerful attack that can break the 9999 damage cap.
Realm Synergy
Now, remember, all the characters, bosses, and stages, are from past Final Fantasy games. This isn't just fanservice, it's enshrined mechanically. If a relic or character is from the game being adapted, it gains Synergy. So they have higher stats, and get more EXP.
This is Cloud if I'm using him in a stage based off Final Fantasy 7. You can tell if something is effected by Synergy if it has a blue aura around it. So while his Organics and Hero's Belt get it, because their items from FF7, the Kaiser Shield does not get Realm Synergy in this case, because it's an item from Final Fantasy Tactics.
Synergy is when combined 3-4 Relics comes into play. With Synergy in effect, a combined 3 star is about as good as a 5 star without Synergy, and a combined 4 star is better.
New Player tips!
Now then, if you've read this OP and downloaded the game, there's a ton of stuff you need to worry about. As such, here's some tips to go through the beginning part of the game and start progressing.
1. Plow through Realm Dungeons: Early dungeons have low Stamina costs and often give multiple Stamina Shards so you'll be able to go through this pretty easy with the free 5 Star you draw and Roaming Warriors. The Orb rewards will be small, so you'll primarily be doing this for the Mythril and Stamina.
2. Once you hit 50 Mythril, do a draw on the banner of your choice: While you'll hear more experienced players talking about "saving" for some particular relic, that's because they have a wide spread pool of relics already and can afford to wait. You, however, can not.
3. Progress as far into each Event as possible: Generally, again, you'll be able to at the very least clear the first "Classic Dungeon" half of each Event off the back of Free 5 star and RW. These will give better rewards then the early Classic Dungeons
4. Keep building up abilities and relics and soon you'll find yourself progressing further and further.
5. Post screenshots of Full Break Crafting, First Ultimate Victory, and first 5 Star Hone in the thread to make us all proud of how far you've come. 
Other Terms.
There's a few other bits of terminology used that I think might be handy to know.
Wall: The effect given by the Soul Breaks "Sentinel's Grimoire" and "Stoneskin II", used by Tyro (the player Self insert character) and Y'shtola (catgirl from Final Fantasy XIV) respectively. They buff Defense and Resistance by 200%, making them very useful effects for High end content.
Hastega: Any Soul Break that casts Haste on the entire parry, increasing their speed.
Medica: Any Soul Break that gives out an AOE healing
Trinity: Wall, Hastega, Medica. Generally having 2 out of the 3 is what's considered the point where you can easily tackle said high end content.
Roaming Warrior At the start of each dungeon, you can pick one of these. Their other players characters, which you can then "Summon" to use their set Soul Break twice per Dungeon. Incredibly handy.
Boostga/Faithga: An party wide Attack/Magic boost. Is increasingly needed for high end content. Luckily these are easily gotten with Roaming Warriors.
X Meta: A description of a Team Archetype in the game. There are several different types of them. A "Mage Meta" team would use multiple stacking Magic buffs to blast away with Black Magic and Summons, while say a "Fire Meta" would focus on Fire elemental abilities and Soul Breaks, often using ones that weaken the bosses resistance to Fire and strengthening your own characters power in the ability, etc.
Buff Stacking
This gets its own section because it's pretty important.
Now, the way FFRK handles buffs and debuffs is different from most RPGs I've seen. The way it works is that each Buff/Debuff has its own ID, based off what stats it effects. So, an ability that reduces the Enemy's attack and one that raises the user's attack would have the same ID, because they both effect just Attack. You can't have two effects with the same ID on the same character, the new one will just overwrite the old one.
So if you use Attack Breakdown, then Attack Break, you'll just end up undoing your Attack Breakdown for the lesser Attack Break. So don't do it.
However, two Buffs/Debuffs that effect different stats will stack. So Attack Buff/Debuff and Magic Buff/Debuff will stack. And here's where things get crazy. Some abilities give Buffs/Debuffs that effect multiple stats at once. These then have their own unique ID. So let's say you have three Debuffs to inflict on the enemy. An Attack one, a Magic one, and one that decreases Magic and Defense. Then you throw on a buff that increases your Party's Attack, then one that increases their Magic, then another that increases both Attack and Magic. All three will stack since all three have separate IDs. This is important to know because high end FFRK content pretty much requires the ability to be able to stack up multiple buffs and/or debuffs.
Helpful Links
Just going to quote this from the last thread.
pichupal" post="465089192 posted:
I can put together a list of some helpful links and stuff, the OP is gonna get crowded if everything is explained.
Just a few I can find pretty quickly are:
Enlir's FFRK Database - Best for looking up just about any detail on a character, Record Dives, SBs, ect.
MisterP Record Keeper PDF - For looking up quick tips for clearing realm dungeons, a great diagram on 6* Crystal competition, damage formulas and details.
Roaming Warrior Search - For when you need to find ANY Roaming Warrior, you'll find it here.
Banner Relic Details by kaonohiokala - For looking up specifics and details on future banner relics.
Kongbakpao's FFRK Event List - Dates and details on current and future events.
Reddit Ability Guide - Good base for explaining what abilities to make to start challenging the Ultimates, including recommended 3 and 4* abilities