In digital burn, there are two kinds of net. Full immersion, and partial immersion.
Partial immersion is slower, less intense, and the user is still aware of the real world (meaning sooner or later they have to eat/sleep)
Full immersion is faster than the real world, quite intense, and leaves the user without any sensation of the real world. This has the nasty side effect that you can starve to death and not notice if you're idiotic enough to not get offline sooner or later.
Digi burn also has new uses for skills, new feats, new advanced classes, new prestige classes, new futuristic equipment, and rules for cybernetic enhancement. (Never be without juice with the new Blendo-thigh 3050!)
It also has information on mega-corps, new diseases and drugs, and profiles of the major national powers.
Still, for the $35 price tag I was a little disappointed. Around 150-something pages of material, and the quality of the paper itself really bugs me (it's tissue thin and a little too plasticy.)