Originally posted by: coolpurvi
I found this story a bit confusing
Adi-shesh is also called Anant-Shesh, Shesh-Naag or Sankarshan. He is the thousand headed serpent on which Lord Vishnu reposes, floating upon the Sea-of-milk (Ksheer-sagar).
He is the first born of the serpents. king of serpants along with brothers like vasuki and his mother is Kadru, a daughter of Daksh Prajapati. He is the one responsible for all the planets and stars to revolve around each other with such precision, the force that in science is called Gravitation. His father is the sage Kashyap.
According to the he was asked by Lord Brahma to bear the weight of the world on his head. Sheshnaag was saddened by the wicked ways of his brothers, and decided to leave them. He repaired to the mountain Gandhmadan and practiced ascetic devotions, living upon air and rigidly observed his vows. He passed his days in those sacred regions, with his passions under complete control.
When Lord Brahma saw the devotion of this serpent, he appeared in person and offered to grant him a boon. Shesh said, "My brothers are wicked of heart. I do not wish to live among them. Let this not draw the censure of the world. I wish to cast off this body of mine and attain salvation. I am very much grieved by the treatment meted out to my aunt Vinta and her son by my mother Kadru, and brother Naags."
Brahma said, "Thou shalt attain salvation in due course. Ask me another boon." Shesh replied, "O divine Grandsire, then please let my heart always delight in virtue and in ascetic penances." The Lord said, "So be it. I shall also confer another honor upon you. You shall be the bed of Lord Vishnu, and bear the great weight of the earth upon your head." Thus, this mightiest of the serpents, holds our mighty planet in her orbit.
Anant-Shesh also incarnates on the Earth along with the incarnations of Lord Vishnu, in order to serve Him.
(from puranas)