A general purpose mobile computer. It has a keyboard (which may include a numberpad on larger models), and is generally equipped with a screen 13"-18" in size. There are many different uses for notebooks, including basic typing/internet uses, video/audio editing, CAD, and gaming. They can be pretty light and also very heavy, with a wide range of battery lives (~3 hours average I'd say). They also can be quite expensive, and sometimes run hot.
A small mobile computer, generally with a less-than full size keyboard. Screens are generally 12" or less. They are equipped with low-power CPUs and other hardware only designed to be used to access the internet/do basic computing tasks on-the-go. They are very cheap (generally less than $400) compared to their bigger brethren, and usually have high battery lives (~5 hours average, I'd say).