I can’t actually get any confirmation of this, but presumably the “S” in the title stands for “Social” as this is a Japanese Mobage game.
To anyone still reading, Final Fantasy Tactics S is the latest Final Fantasy branded browser-based mobile game (after Final Fantasy Brigade) and uses the setting, rather than the gameplay, of the Final Fantasy Tactics Advance universe. Mobage titles aren’t downloaded apps but are instead run entirely within your iOS/Android web browser after signing into the Mobage service. To keep download speeds/sizes down there’s no music or sound effects, you can somewhat alleviate this by playing the FFTA2 OST while gaming, my personal choice would be this track.
The plot, if the game can be considered as having one, is to build an ace clan full of tough people and defeat all oncomers – monsters, bosses and other clans alike. There is a bit more to it than that – there are bits of dialogue before you face bosses and other events but the game isn’t concerned about seriously fitting itself within established Ivalice canon.
As is to be expected from a game of this type, there’s not a lot of difficulty or real strategy here; battles are played out on a rather Ogre Battle like field and entirely automated after you’ve set up your team on the pre-battle menu screen. The game essentially boils down to completing quests by using your team to defeat monsters/other gamers clans, then using the money/materials earned to boost your teams equipment and increase your clan rank. Completing these quest nodes naturally opens up further quest nodes in the area so you can do it all again for shinier equipment.
There is a reason to be concerned about you clan rank - more powerful clans can recruit more members and can complete more quests before becoming fatigued (fatigue can be replenished with paid for elixirs or in game).
This is where the money comes in – players can buy items and super rare characters (defined by job type, there are no unique units as far as I’ve seen – edit: Just had my backside kicked by Agrias) to give them an advantage in the game. It’s no better or worse than any other game of its type in this regard.
Final Fantasy Tactics fans shouldn’t feel as if they’re missing out; this isn’t an exciting Japan exclusive addition to their favourite series – it’s another throwaway mobile game… but then again that’s all it’s designed to be. It’s a nice enough way to while away a post office queue or a coffee break, it’s not intended to be a “real” game for dedicated RPG fans to spend the next 100+ hours with and I can’t really fault it for doing exactly what it sets out to do.