These satellites are the ones used for relaying TV and radio communications round the world. The signal is transmitted from the ground, received by the satellite which then transmits it to the receiver on the ground. They are now a vital part of our lives.
Almost all international communications are relayed by communication satellites.
This animation shows two satellites, one geostatiuonary (orange) and one non-geostationary (light blue).
Geostationary satellites 'hang' in the sky above one point on the Earth's surface - they have a period of revolution equal to the rotation period of the Earth. In other words they take one day to make one revolution. This means that ground based aerials do not have to be moved to pick up the transmissions from them.