The wall was built over 2000 years ago by Qin Shi Huangdi, China's first emperor. While I was working at the Science Centre last year, the IMAX theatre was running a show called The First Emperor of China, and it was a documentary on his dynasty. He standardized Chinese writing, bureaucracy, scholarship, law, currency, weights and measures. China was also named after him, Qin, pronounced Chin.
He built, or rather extended the old fortification walls along the northern parts of China, thus constructing the Great Wall of China. In Chinese the wall is called "Wan-Li Qang-Qeng" which means 10,000-Li Long Wall (10,000 Li = about 5,000km). After centuries of rebuilding, it stretched over 6,500km long. Its thickness range from 4.5 to 9m wide, and some parts of the wall are 7.5m high. Somewhere between 2 to 3 million people died building it over the centuries.
An army of around 1 million men were stationed along the wall, and it served as the first line of defense against the Hsiung Nu tribes from north of China. Today some parts of the wall are preserved and opened for tourists, but some parts of it are impossible to repair.
The Great Wall of China cannot be seen from the moon though. That's a myth. Funny how I remember reading that in one of my history textbooks in school back in maybe 1994. It could be visible from space at close range, but even then one has got to know where exactly to look. It also depends on visibility conditions.
Well.. who would want to see the Great Wall of China from space when you could actually be there? :)
(Or at least in your dreams, haha)