Most common geostationary satellites are either weather satellites, communication satellites relaying signals between two or more ground stations and satellites that broadcast signals to a large area on the planet. The best example of the latter is satellite TV.
Geostationary satellites are in a geostationary orbit around the planet's equator at an altitude of approximately 22,236 miles above sea level, and they travel at 1.91 miles per second in the same direction the Earth is turning. This orbit is a popular choice for communication satellites operators and their users because when looking from the ground to the sky, the satellite is always in the same position. This allows the satellite antenna on the ground to receive a signal while always pointing the same way.
The downside of satellites in a geostationary orbit is their lack of coverage of locations near the Earth's geographical poles. At such an extreme latitude, the satellite would be near the horizon and easily obscured by nearby hills. Another downside is the increased cost of launching a satellite because the rocket requires more fuel to reach such a high altitude compared to other common satellite orbits. Also, because of their high altitude, any signal sent from Earth to the satellite and back would take at least a quarter of a second to complete the trip, which is not insignificant for latency-sensitive applications.