Basically, what it is is a problem solving group. They are handed projects by scientists or other people... and instead of using an expensive supercomputer to do they're calculations quickly, they use tens of thousands of peoples home computers around the world. The legitimacy of the uses im not too sure about, it works by sending your computer a chuck of data and some instructions of what to do with it. Normally this would involve your computer simulating this data many times with randomisations to test different outcomes. All this is handled my the program, you just install, select which projects you would like to contribute too, how much cpu you want to allow to use, then let it go.