When you say tablet, I am assuming your refering to the ability to take a pen-like device and write on a electronic device that would send input into a computer, am I correct?
I've used them for YEARS. They've been around, one way or another, for as long as people have been doing art on computers.
Normal ones (the ones I prefer) use a seperate pad that sensitive to the "pens" that you use. There are others that are clear and can fit over a screen, but I don't like those and they are very expensive.
The best brand and the ones with the best prices are from
Wacom
There are others that are cheaper but they have sensitivity issues and it may be tough to draw perfectly straight line or do it accurately. You can get wacom tablets from $75 in some places upwards to several thousand dollars, depending on the size and features you want.
They plug into your computer via USB (like newer mouses) and are good for Macs or PC's.