Laptops have a cd/dvd drive; netbooks don't
Laptops can be up to 20" screen size; netbooks are small and easily portable
Laptops battery life can be as little as one hour; netbooks can be as much as 9 - 11 hours
Laptops have full size keyboards; most netbooks do'nt
Laptops can have very good screen resolution, some may be powerful enough for gaming and video editing; most netbooks have inferior screen resolution (fine for surfing the web, emailing, word processing) and less powerful workings.
If you intend to use the machine as a desktop replacement and carry it only now and again, get the biggest screen you can.
If you want to carry it about all the time and just use it to access the web and email, get a netbook. They are specifically designed to be small, lightweight and easily portable.