LEO satellites orbit between 2,000 and 200 kilometers above the earth. LEO satellites are commonly used for communications, military reconnaissance, spying and other imaging applications.
Most of the man-made objects orbiting earth are in LEO. Satellites made for communications benefit from the lower signal propagation delay to LEO. This lower propagation delay results in less latency. Being closer to the earth has an obvious benefit for many types of earth observational satellites by resolving smaller subjects with greater detail.
LEO satellites are generally less costly to place as they require a great deal less rocket power to place. As compared to geosynchronous orbiting satellites at 36,000 km, LEO travel through a much denser atmosphere and thus experience far more aerodynamic drag. This means they require more power to travel at higher speeds and make corrections to maintain their lower orbits. Where geosynchronous satellite orbits in time with earth rotation at about 3.06 x 103 meters per second an LEO satellite might travel at 7.78 x 103 meters per second, orbiting many times a day. The ISS orbits at 400 km and makes a full revolution round the earth about every 93 minutes.
LEO satellites have a much smaller field of communication with earth than a satellite at greater altitude. They also have a faster rotation around the earth. These factors necessitate a constellation of satellites to operate in concert for some applications. A constellation is a group of satellites working in concert, spaced in order to provide the required coverage.
The range of LEO satellites ends where medium earth orbit (MEO) begins at 2,000 km. MEO extends to the reaches of geostationary or geosynchronous orbit.
Watch an introduction to LEO and GEO satellites: