Google will tell you what one is.
In my words - it is a cute little box that acts just like a little digital photo frame. But this photo frame has a brain, and wifi..
It can show you the latest news reports, or the weather, or the time, or a webcam, or stream music from the web, etc. And you can touch the screen, and make it do stuff. Like play a game of chess. And you can pick it up and shake it, and tilt it, and the screen will respond. Cool, eh?
So you set it up, put it somewhere, and glance at it every now and then. Just like a photo frame, but with much more interesting and relevant information.
Whilst this is cool, it is not why I bought one. I bought one because I can code stuff for it. Now it is very cool.
So buy one. If you are in Australia, like me, then try priceusa.com.au (they are only sold in the USA currently).