Yes. You beat the game on a character in Normal, after getting to like ~30 or whatever (dunno, made up number). Then you can play through again from the begin

Yes. You beat the game on a character in Normal, after getting to like ~30 or whatever (dunno, made up number). Then you can play through again from the beginning doing the exact same thing, with tougher monsters, in Nightmare. Beat nightmare and you're at 40~50 or whatever. Repeat again on Hell, at which point you're probably 60. Repeat again on inferno. So basically you play the same game over and over again.