A netbook is a lot smaller and lower powered than a notebook.
Netbooks typically have about 10inch screen maximum, a slightly smaller than standard keyboard, no CD/DVD drive and a low power processor designed to give best battery life, not best performance.
Notebooks (also called laptops) can generally be considered to be a replacement for a desktop PC with most of the same features and performance, they have larger screens, full size keyboards and will have a built in CD/DVD drive.