So, it's a card game but with only girls. Yeah, that's about it.
The game is pretty easy to play since it doesn't go with energy cards or sacrifices for higher tier cards and different fields for different card types, but they do limit the cards on the field by a size stat.
The site pretty much explains most of it.
The game has payed content, but paying for cards only means that you are trying your luck to get a card without much effort, since all cards can be crafted, except a few cards that are obtainable via other methods (winning them on the respective dungeon, or on a special type of trade).
There the rest of the payed content only makes your life easier, like higher change of card upgrade, or get more training points.
They seem to have time limited cards that you can pay for (again, trying your luck), but you are also able to craft them, for free.
Basically, poor fag grinds and crafts (there is no success rate on crafting which is awesome), rich fag gambles.
Being web browser and flash, makes it playable on many devices, which some seem to like that.
(Sorrow doesn't like things like referral spam, so I won't spam your FB with it, will just leave this in here.)