http://www.youtube.com/watch?v=gTrO3vj2heQ
Neither of these people are very good.
Anyway, you win either by filling up your opponent's side or by clearing 200 gems first. You can add more rows whenever you want to give yourself more gems to combo, at the risk of overflowing yourself.
Characters are based on tarot cards and they influence how gems come down on your side and how they're dropped on the other side when you clear blocks. Tons of characters.