They are used for gathering information about the earth, eg weather, and for communication.
Geostationary satellites orbit at about 35,000 km above the ground. Remember Kepler's laws? They apply to satellites as well.
Thus the higher the satellite, the longer its' period of orbit.
In this animation the yellow dot represents the satellite.
Original animation from Bradford telescope project. Satellite added by S.Mack & Gif Movie Gear
