| the "fastest" satellites orbit the earth every 90 minutes, starting around 130 miles up. any lower than that, and they encounter enough atmospheric drag that they fall down.
as satellites go higher, they actually travel slower, and also take longer to go around.
at around 23,000 miles up, satellites over the equator take 24 hours to orbit the earth, and are in what are called geosynchronous orbits.
then, when you get up 240,000 miles, it takes a month to go around.
there are hundreds of satellites we're put up. why don't they run into each other? because there's lots of space. let's suppose you drop 50 people around the united states. maybe one in the middle of each state. how likely are they to run into one another? not very.
to that, you have to add that the US occupies a very small part of the earth, and is 2 dimensional, rather than 3. that said, the important satellites do have thrusters, and are occasionally moved a bit to insure that they don't run into each other.
Saturn has 2 moons that share the same orbit.
if you read the links, and understand the dynamics of the moons, then you'll have a pretty good handle on why the higher satellites travel slower, but take more energy to get them where they are. | 
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