It's basically your standard Diablo 2 Clone done right. The game is a great ARPG albeit the story is a bit lacking. The story does however improve on the original. Torchlight was a good game too but if you played it on console I could definitely see it getting frustrating.
Online play is the same as Diablo was. Log into their systems, find a game, join the fray. The creator of the game is the master where the quests are. So if you're a higher level than your friend, had the lower level friend create the game and then you join so you can work on their quests.