Old Summer Palace was once a complex of palaces and gardens built in the 18th and early 19th century. It served as an area where emperors of the Qing Dynasty would handle government affairs. In October of 1860, during the Second Opium War, British Commisioner Lord Elgin ordered the destruction of the palace in response to men that had been tortured and executed in late September.
What remains are mostly European-style buildings, and very few Chinese-style buildings. Plans to rebuild are out of the question, because any reconstruction would destroy a part of Chinese history. What they are and have been doing though, is restoring old vistas and filling lakes. And what remains of Chinese buildings have been refurbished.
You still have all the vendors and souvenir shops here, but there was one booth in particular that was really fascinating. A man was making dragons out of melted down sugar, shaping everything with a small stick. Really cool.
This is but a scratch on the surface to what Old Summer Palace is about. If you want to read more about it you can check out wiki, or cross reference other sources you find elsewhere.