Atlas was a second-generation Titan god, from a patrilineal perspective. He was the child of the first-generation Titan Iapetus and Clymene, a daughter of Iapetus’s brother Oceanus (an Oceanid). With his Oceanid aunt Pleione he sired the Pleiades, the Hyades (whose mother is also named as Aethra, another Oceanid), and Calypso. With Hesperis, daughter of the Evening Star Hesper, he produced the Hesperides. Atlas’s brothers were Prometheus, Epimetheus, and Menoetius.
The mythographer Hyginus reports that as a consequence of his leadership role in the power struggle between the Olympian gods and the Titans, Zeus punished him by placing the heavens on his shoulders. Atlas was said to live by the garden of the Hesperides, on the westernmost shores of the river Oceanus, in Libya, or in the distant North (or East), where the Hyperboreans resided.
Hercules enlisted Atlas’s assistance when he went to fetch the apples of the Hesperides: Hercules asked Atlas to retrieve the apples in exchange for giving Atlas a break from holding up the heavens by shouldering the burden himself. But Atlas, apples in hand, attempted to protract the reprieve from his onerous task by offering then to deliver the apples to Eurystheus, at whose behest Hercules was performing the task. Hercules tricked him into taking the heavens back by asking him to hold up the burden momentarily while he looked for a pillow to cushion his shoulders. Needless to say, Hercules did not resume the burden. Atlas also played a role in the saga of the hero Perseus. As retribution for not having offered him hospitality, Perseus, holding up Medusa’s severed head, transformed Atlas into the mountains that still bear his name.
Sources: Classical Mythology A to Z