From Diablo Wiki
Single Player is different from multi-player by many ways, Closed Battle.net is for playing with other's like it was single player, besides the fact that you can get help, instead of playing by yourself.
Open Battle.net is just like single player, you use the same characters as you did on single player, and you can also use cheats and hacks.
Now on to single player, you can create any character you want, and name it anything, but you can't name two characters the same exact name. When you get into the game, prepare for it to be harder, and for you to be alone. But a trick to level faster, but monsters being harder, you must type /maxplayers 1-8
After you do that, monsters will be stronger, but give you more experience.
Besides this, Diablo is the same on single player, Open Battle.net (besides cheats and hacks) and Closed Battle.net, just by yourself.
Klank 21:53, 25 July 2011 (CEST)