...you've never heard of them because they cost thousands of dollars (iirc) to build/maintain. They are HUGE, loud, expensive to run, and put a lot of draw on the power grid.
If all you've been running is a stock PC (bought from Dell or something) and haven't touched the inside of a computer (which I suspect), stay FAR away from them. I don't even know how to set one up (very few do, which is why they are expensive), or run one...and I'm a hardcore overclocker...
~Gooda~