As has been said, a laptop (sometimes known as a notebook) is a full "PC" that normally runs Windows, and has a CD/DVD player. Screen size is usually 14" or above.
A netbook is a smaller device with a screen size 8" or so.
Some run Windows but others run a different operating system called Linux.
The cases are not normally big enough to have CD/DVD drives in them.
Some of them (all of them?) use a "solid state" hard disk (no moving parts)
I guess a netbook is part way between a hand held PDA and a laptop.
Small enough to carry round, but large enought to actually work with.