A Low Earth Orbit (LEO) is generally defined as an orbit below an altitude of approximately 2,000 kilometers (1,200 mi). Given the rapid orbital decay of objects below approximately 200 kilometers (120 mi), the commonly accepted definition for LEO is between 160 kilometers (99 mi) (with a period of about 88 minutes) and 2,000 kilometers (1,200 mi) (with a period of about 127 minutes) above the Earth’s surface. Because it’s so close to Earth, satellites must travel very fast so gravity won’t pull them back into the atmosphere. Satellites in LEO speed along at 17,000 miles per hour (27,359 kilometers per hour)! They can circle Earth in about 90 minutes.
Orbit between 99 miles and 1,200 miles
LEO has been used for both military and aeronautical purposes. Military rocketry and missiles have long taken advantage of this orbit to launch missiles and rockets over long distances. A missile launched in low earth orbit follows three stages. First it would launch into a suborbital path using its engine. The second stage would be where the thrust and momentum created would allow the missile to reach cruising speeds. In the final stage, the influence of gravity brings it back to Earth towards it target. In space flight the majority of human spaceflight occurs here. Right now the cost of human spaceflight is astronomical so most space agencies are funded by governments and need to work within set budgets for missions. This is why Low Earth Orbit is still the destination of choice for missions. Most satellites, the International Space Station, the Space Shuttle, and the Hubble Space Telescope are all in a LEO.
Space Junk 
The LEO environment is getting very crowded. The United States Space Command keeps track of the number of satellites in orbit. This is a graphic display of the objects in low earth orbit. According to the USSC, there are more than 8,000 objects larger than a softball now circling the globe. Some people worry about the number of items now in low earth orbit. Not all of these things are working satellites. There are pieces of metal from old rockets, broken satellites, even frozen sewage. At 17,000 mph, even a small bolt can hit a space shuttle with the impact of a hand grenade.
AcqLinks and References:
-  Website: Wikipedia – Low Earth Orbit
-  Website: TheTech.org – Low Earth Orbit
- Website: Rocket and Space Technology