Size and functions.
Laptops have bigger screens so weigh more and have larger keyboards, usually have cd/dvd writer; a card reader and other functions.
Many laptops can realistically be desktop replacements.
Netbooks are small and lightweight, designed for browsing the web on the move, sending emails and so on.
The keyboards are generally too small for prolonged typing and the screen too small or video editing.
The generally don't have a dvd player and other functions although they have a few usb ports.