Notebooks and Laptops are portable computers but there is a minor difference between laptops and notebooks.
"Laptop" computers are somewhat larger than notebooks and will accommodate a built-in disk drive unit and, optionally, a floppy drive unit. That is, a CDROM drive is either built into the unit or the unit has a built-in bay that will accommodate a removable CDROM drive.
"Notebook" computers, on the other hand, will allow you to attach, via cable, a CDROM drive. Since the drive is not built into the unit, notebooks are smaller and lighter than laptops. Hence, a notebook is an ultra-small laptop. Another way to look at it is a notebook is about the size of (or a little larger than) a real binder notebook.
In short you can say that:
Laptop: A portable computer small enough to use on one's lap.
Notebook: A light, portable computer that is generally thinner than a laptop.
Even according to Webopedia, laptop computers are more frequently called notebook computers, though technically laptops are somewhat smaller in size than notebooks.