Kingdom Hearts II starts off with you playing as Roxas. It's more of a training level than anything else, and it also shows you exactly how Roxas is related to Sora. And how Axel is related to him, too.
When you meet up with Sora & the gang, the game plays exactly how the first one does, with a few twists.
The Triangle button is now a 'Reaction Command' and causes Sora (Or Roxas, at first) to do a breathtaking move, either with his partner or his enemy.
There's also new forms you can transform into, but I personally went through the whole game without using them. You need to give up Donald and/or Goofy in order to use them.
More to come soon!