Probably the most famous people with the title Khan were the Mongol Genghis Khan, and his grandson Kublai Khan who founded the Mongolian dynasty in China, known as Yuan Dynasty.
This title was probably first used by Rouran; maybe the Xianbei used it earlier. At that time it was pronounced Khaghan. The gh sound later weakened and disappeared becoming Khaan in Modern Mongolian. The Secret History of the Mongols clearly distinguishes Khaghan and Khan: only Genghis and his descendants are called Khaghan, other rulers are referred to as Khan.
The title Khan was also used in the Seljuq Islamic dynasties of the near-east to designate a head of multiple tribes, clans or nations, who was below an Atabeg in rank.
See also: Khong Tayiji, List of Mongol Khad