Netbooks are a smaller laptop usually powered by the new Atom processor. That have 3G built into them directly. They are meant for users who want to check email and webpages, but not for users who expect much else.
Some netbooks have more power behind them than others but they usually cost a lot. For instance Sony makes some netbooks with some really beefy power but they are close to $1k. Which makes little to no sense since you can buy a godly laptop for that price and just get a free aircard.