Genghis Khan, originally known as Temujin, was the son of a tribal chief in present day northern Mongolia. Khan united his tribe with several other surrounding tribes in 1206 when a kuriltai (tribal council) recognized his leadership. Genghis Khan began the Mongol conquests in 1209 with the attack on the Xi Xia kingdom in northern China.

So to answer your question, the 'Mongols' were just a series (dozens) of nomadic tribes in the Mongolian plateau prior to Genghis Khan's unification. There was constant fighting and raiding between these groups on the central Asian steppe. By the time Khan united these steppe tribes, he ruled over roughly 1 million people.

Sources:

Grant "5,000 Years of Combat"

Ratchnevsky "Genghis Khan: His Life and Legacy"