Sage Durvasa had given a divine mantra to Kunti. If she invoked any god by chanting that mantra, he would appear before her and bless her with a son just like himself.
After her marriage to King Pandu, Kunti used the divine mantra and gave birth to Yudhishthira.
She then chanted the mantra to invoke Vaayu, the Wind-god.
Vaayu appeared before Kunti and blessed her with a son named Bheema. Bheema had superhuman strength.
The five sons of Pandu were known as the Pandavas. The hundred sons of Dhritarashtra were known as the Kauravas. Bheema was the strongest and the naughtiest of all the brothers.