Taken from Wikipedia:
"A tablet is a computer input device that allows one to hand-draw images and graphics, similar to the way one draws images with a pencil and paper. These tablets may also be used to capture data or handwritten signatures.
It consists of a flat surface upon which the user may draw an image using an attached stylus, a pen-like drawing apparatus. The image generally does not appear on the tablet itself but, rather, is displayed on the computer monitor."
This is the one I have: Wacom Bamboo Pen & Touch - Digitizer, stylus - Wired - USB
I hope that helps :)