
As for what it does... it connects to the internet and downloads a "data unit" to process. Whenever your computer is idle, the program will work on the data unit in the background. When it's finished, the completed unit gets sent back to SETI, and a new one is downloaded.
It can be set to run in the background constantly, or just as a screensaver (I have mine on screensaver mode, but of course my computer's on while I'm at school or sleeping...)