A tablet is something that you draw on. You use a special pen. As you draw on the tablet, it moves your mouse like you are drawing on the screen. Even better, it knows how hard you are pressing and many brushes on the screen adjust to the pressure you use on the tablet.
The latest Intuit tablet (I think it is the Cintique) is actually a little monitor so you can draw right on top of your picture. With other tablets, you look at the monitor and move the pen on the tablet.
Personally, I haven't found it to be much better than using a mouse. I think the big difference is that I don't ever draw/paint with it. I just use it for detailed touchup work and I can do that reasonably well with a mouse anyway. Most serious photos/graphic artists love theirs, so don't let me discourage you.