It is generally considered to comprise the Mongolian Plateau and some adjacent territories, although its exact extent has been changing over the course of history—particularly since the rise of the Mongol Empire in the 13th century.
The modern area that the Mongols live in approximately includes: the modern state of Mongolia; the Inner Mongolia Autonomous Region, along with Dzungaria in Xinjiang and parts of Manchuria, Qinghai, and Ningxia in China; as well as the Buryatia Republic and a few smaller territories in Russia.
It originated from the Mongol heartland in the East Asian Steppe, when Genghis Khan united the nomadic tribes and became the first Khagan of the Empire in 1206.
Later Kublai Khan (Genghis Khan's grandson and founder of the Yuan dynasty) shifted the political center of gravity from Karakorum in the Mongol heartland to Khanbaliq (modern Beijing) in North China, but the Mongolian Steppe remained a significant place for the Mongol rulers.
With the exception of Buryatia and some neighboring territories under the Russian Empire, the Qing dynasty ruled most of the Mongol heartland for over 200 years.