"The ISS is by far the largest artificial satellite that has ever orbited Earth. The ISS serves as a research laboratory that has a microgravity environment in which crews conduct experiments in biology, chemistry, medicine, physiology and physics, as well as astronomical and meteorological observations. The station provides a unique environment for the testing of the spacecraft systems that will be required for missions to the Moon and Mars. "
From Wikipedia
http://en.wikipedia.org/wiki/International_Space_Station