hmm... Let me explain. When you enter an instance, you enter a Cave for example and a new lvl starts, new mobs spawn, new bosses spawn, and when you get out of the instance, you can get in again and kill the same mobs. It is easy to understand
For exp, I 'd suggest Warlock = Good curses and you have a demon to help you killing, Hunter = ranged attacks and a pet to tank for you + traps to keep distance, and Rogue = You can avoid hard mobs and move faster.