Here's a breakdown from our gameplay producer Mike Wang:
http://www.easports.com/blogs/nbalive/inthepaint/post/slug/gameplay-defense
Rookie = For kids and total newcomers to basketball games. My 6 year old son can compete in this one.
Pro = For the casual basketball fan. The AI is a little tougher but defense is toned down quite a bit to allow for easier shooting and dribble penetration (read: more dunks)
All-Star = For the “serious” hoops gamer. AI is pretty tough, especially when playing with a non-elite team. Most experienced gamers will find a nice challenge here.
Superstar = For the hardest of hardcore. AI is dialed up even more. Expect to concentrate hard on every possession and have almost flawless execution to win.
So make sure you have the difficulty set to match your expectations to maximize your enjoyment of Live this year.