The Great Wall of China was a structure built in China on Earth to, as Kathryn Janeway pointed out in 2375, "keep people out". It was completed by the first Qinemperor in the 3rd century BC, who connected the walls built by a previous dynasty. He did so to prevent an invasion by nomads to the north. The full wall was 2,400 kilometers long, with a median width of 3.8 meters.
According to Neelix and Janeway, before the 22nd century, it was one of the only man-made objects that could be seen from Earth orbit with the naked eye. (VOY: "11:59")
The statement that the Great Wall is the only man-made object that can be seen from space is a well-known false urban legend.  According to several NASAastronauts, the Wall itself is not visible from medium to high orbits (180 miles from the surface). While it may be visible from lower orbits (around 130 miles from the surface), other man-made objects are clearly visible as well at that height as well, even more so than the Great Wall. NASA space shuttle astronaut Jay Apt has stated that "Although we can see things as small as airport runways, the Great Wall seems to be made largely of materials that have the same color as the surrounding soil. Despite persistent stories that it can be seen from the moon, the Great Wall is almost invisible from only 180 miles up!" ("National Geographic", Nov 1996)
The actual Great Wall is currently considered to have an overall length of 8,850 kilometers, with 6,260 kilometers being actual wall.