Wikipedia There is more information available on this subject at Geosynchronous orbit on the English Wikipedia.

A diagram of a Geosynchronous orbit

A geosynchronous orbit is an orbit around a planet with an orbital period matching the planet's sidereal rotation period. A satellite having a geosynchronous orbit will orbit relative to the same, fixed position on its parent body (e.g. Earth) at exactly the same time each day. The term geosynchronous can be used to describe any orbital pattern with a period equal to that of its orbiting body, and in the same direction as the parent's rotation, but is specifically used for orbits with no inclination and a semimajor axis of equal length to the minor axis (i.e. circular). This means that the orbiting body essentially appears to 'hover' or hold over a specific location during the entirety of its orbit. This also referred to as a geostationary orbit.

Orbital Defense Platforms in particular make use of geostationary orbits to protect certain ground regions from attack; Cairo Station, for example, orbited directly over the city of Cairo, taking its name. The Athens Station also orbited above the Greek city of Athens. Other satellites, predominately telecommunication satellites, orbit in traditional geosynchronous orbits, holding over the same point during every rotation of the Earth. The terminus stations of Space elevators are also anchored to the geosynchronous orbit.