From Earth, a satellite in geosynchronous orbit appears to "hover" over one spot on the Equator. That means a receiving dish on the Earth can point at the satellite at one spot in the sky and not have to "track" its motion.
The satellite isn't motionless, though. It's in a very high orbit where it circles the Earth once a day, matching the Earth's rotation on its axis.
There are many satellites currently in geosynchronous orbits. The weather satellite pictures (GIF, 60k) we see on the nightly news are constantly fed to earth-bound receiving dishes from geosynchronous satellites like the GOES weather satellites (Visit the GOES Weather Imagery Site). So are most cable TV channels.
NASA also uses geosynchronous satellites to relay communications and data between spacecraft, such as the Space Shuttle and the Hubble Space Telescope, and control centers on Earth.