Just the size .. really. Netbooks are like .. book size.
They also usually have fully pre-loaded software .. some of it propriatory to the manufacturer. Thay are not as flexible and powerful as most laptops, but are small and light. Too small for some, tho.
Netbooks are up to 10" screen size, have no DVD drive and usually have an intel atom processor, and a more compact keyboard size (on some netbooks the keyboard are so small they are difficult to use) ...all to save power and weight - some netbooks have 10+ hour battery life(but not all do..be careful with this)
Laptops can have up to 17" screen, usually have DVD or BLU-RAY drive, can have a full size keyboard, some are widescereen now and have full size keyboard with the numeric keypad on the right as well! - some of them...like a MacBook pro 17" weigh a ton and are not that easy to carry....depends on what you need from your portable machine...
a notebook is usually a basic, less powerful cheaper smaller less quality, less speed,limited more to upgrade. limited version of a laptop just to carry around when your out and about. A Laptop is higher up the scale with usually bigger memory,faster processor,larger hard drive,built in dvd rom larger display. depends what you need it for.
They're not supposed to call them 'laptops' any more because it suggests you can use them on your lap, which is not safe to do in case they overheat (and they often do).