If you mean what it is used for, its most common uses are for weather forecasting, and communications, like satellite TV and radio.
A geostationary orbit will keep the satellite in one area relative to the surface
they fly in air
A geostationary satellite appears to stand still to a viewer anywhere on the earth, not just on the equator.
15000km
A satellite in a geostationary orbit goes around the earth at the same angular speed that the earth spins on its axis. So to someone on the earth the satellite looks like its not moving because its always at the same location above the earth. These satellites are used for communication purposes. For example, TV stations can reflect their signal off of such a satellite by aiming their transmitter at it. They don't have to change their aim because the satellite is always there. Satellite dish receivers can easily pick up the reflected signal just by aiming the dish antenna at the satellite and again you don't have to change this aim because the satellite is always there. The satellites have to be in a circular orbit above the equator in order for it to be put in a geostationary orbit.
Geostationary satellite
Aryabhatta
A geostationary satellite does not trace a path over the surface of the earth because that is what geostationary means - the satellite is stationary over a point on the Earth.
A geostationary orbit will keep the satellite in one area relative to the surface
Geostationary satellites are in an orbit that's 22,282 mi (35,786 km) above the surface of the Earth. For more on Geostationary satellite orbits, visit http://www.idirect.net/Company/Satellite-Basics/How-Satellite-Works.aspx
they fly in air
No. A geostationary satellite appears to be stationary in the sky, which means not moving. This is a big part of the reason why it is referred to as a geo'stationary' satellite.
A geostationary satellite appears to stand still to a viewer anywhere on the earth, not just on the equator.
It is used for communication plates where geostationary satellites can't reach, in polar areas. Russia uses elliptical satellites where the geostationary satellites can't reach.
It should be possible to have the perfectly geostationary satellites. But it is not possible. Perfectly geostationary satellite can not be made. Such satellite is pulled away by other objects like moon, sun and other planets from different directions. This force changes the velocity of the satellites.
Aryabhata in 1975
15000km