Khan (title)

Khan means "ruler" or "commander" in Turkic and Mongolic Languages.
It may be from an Altaic language, probably a Mongolic language like Ruanruan.
Presently, khans exist mostly in South Asia, Central Asia and Iran. 
A female khan is called a Khatun or Khanum.
Khagan means 'Khan of Khans' but is often (incorrectly) 'shortened' to Khan. This is similar to how the Persian "Shahanshah," 'King of Kings', is usually simply called the Shah. Genghis was also called the 'Great Khan' like how Ottoman Padishah was called the 'Great Sultan'.
Khans rule over Khanates. Khagans rule over Khaganates.
Name.
Khan , Khan is a popular South Asian last name (e.g. Salman Khan).
Khan now has many equivalent meanings such as commander, leader, or ruler. It is sometimes translated as king or prince (but khan is a bit different). Originally khans led small tribes.
Variations.
There have been several variations on the title used in modern times. "Khan Bahadur" or "Khan Sahib" was a title of honor, granted by India to Muslims who had done major deeds of valor, or service to the nation or government. The Hindu counterpart for this title was "Rai Bahadur" or "Rai Sahib". This title was normally granted in addition to other awards and decorations and added either before or after a person's actual name as a prefix or suffix.
Mongol Empire.
It was used by the Mongols in their Mongol Empire.
The title "Khan" became well known when Mongol Temüjin conquered most of Eurasia and formed the Mongol Empire. It was the greatest land empire the world ever saw. 
He is often called Genghis Khan. But his real title was khagan. 
The Mongol empire branched into the Ilkhanate of Persia and the Chagatai Khaganate of Central Asia.