The main purpose of this game is to collect cards and step those cards up and level them. You can then use these cards to participate in events or battle other players with them. You collect these cards in Attendance, Events, and the Cupid. You level them up in feeding them other cards. You step them up by combing at least 3 of the same cards.
That's the basic gist of the game; however, it can get a little more complicated than that. Every tab on the sidebar will (try to) explain how the game works in its entirety.