As Gr3atness said, you can't advertise gambling sites in some American locations. The .net sites are officially training sites.
When you download the .net software you can play play money games with the .com players. It's essentially the same network, you just don't have access to the money games.
On PokerStars .net players cannot have a personal avatar, and are marked with a little white star. I don't think they can play freerolls
(but I'm not sure).
Really, there's no reason to be a .net player instead of a .com player.