There are 4 difficulty levels.
Normal, Nightmare, Hell and Inferno. 4 acts each.
It's different from D2 in that the dungeons & monsters are different every time, and they don't always spawn in the same place.
You could hit one monster in one area one time, but if you go back, it may not be there, or it could be a totally different monster.
When it comes to an Elite monster, you'd better watch your back.
The Elite's I hate (don't know the actual names)...... I call them the chain gang! 3+ monsters tethered to a flaming chain, that leave a fire trail in their path.
They triangulate and move in a way that's very difficult to get hits in. You get hit by that chain??........
Jaydip should know them by now EDIT---
One more thing...... After reaching that lvl 60 cap, zero experience! Well, untill they raise it or come out with an Expansion.