Space debris is made up of a growing amount of inactive space hardware in orbit around the Earth as well as fragments of spacecraft that have broken up, exploded, or otherwise been abandoned. This includes the burned-out launch vehicle upper stages, dead or inactive spacecraft, and other objects ranging in size from as big as an automobile to microscopic dust that may pose a threat to active space missions.
The ESA has posted some illustrations of the evolution of the space junk situation. "Space debris have been recognized as a potential problem," said the ESA. "Even though the current space debris population may not represent an immediate and excessive danger, the risk of collision with debris is continuously growing. Now is the time to take action to preserve the commercially valuable space environment for future space users."