Basically, it is a little computer about the size of a credit card that runs linux, has network connectivity, and general purpose input/outputs on it. Great for people who want to do hobby type stuff.
It cost $35 for the one I bought (they have a slightly cheaper one for $25). I will describe my current setup as I begin to set things up. Currently, I have a webserver running to I can access my network storage devices remotely and a torrent application running so I don’t have to leave my main computer running. It draws around 5W of power so quite different than my main computer (up to 300W).
Simply mounted on the wall in my furnace room.
First project is a temperature data recorder. The plan is to utilize the cat5 cables already wired in my house to put temperature sensor on all 3 levels, record the data periodically, and provide a web interface to see a graph of it. Totally useless, but will get me back in the game of doing input/output and force me to get the basics set up on the raspberry pi (webserver, database, programming).