|MadSci Network: Astronomy|
You're right, there are literally thousands of satellites out there. Actually you have great timing, I just found a site, Liftoff Home, where you can check where they all are.
All those satellites are in pre-defined orbits, and actually sometimes a few do collide. Usually, these are the older ones in decaying orbits though. Several of the shuttle missions had to actually adjust their orbits to avoid "space junk."
Hopefully this answered your question.
[Moreover, because the orbits of most satellites are known, when a new satellite is launched, it can be placed in an orbit so that it does not collide with any of the known satellites. Moderator]
Try the links in the MadSci Library for more information on Astronomy.