Well.. my laptop weighs 1KG, has no disc drive internal.. but it's just classed as an ultraportable.. still CoreDuo, 2GB RAM, 12.1" screen etc.
The main defining thing is that they are really small and portable... usually smaller than most laptops and obviously much cheaper. A netbook has high enough spec to work for what it's designed to, but outside of that, it doesn't compare to a laptop. (They sometimes have the new Intel Atom chip - low power etc)
If you just want something to go on the internet from, and nothing much else, and you want it to be really small and portable.. a netbook is perfect.