They are celestial being in resplendent forms. None of the gods could wed then because they did not undergo puriflcation. Therefore. they became women of easy virtue and wives of Gandharvas. They are also presented as a reward to yi heroes fallen in battle. Some of the important Apsaras are Menaka, Rambha and Thilothama, who are sent by Indra to shake the virtue of penitent sages.
References :
1. Kohli, Surindar Singh, Dictionary of Mythological References in Guru Granth Sahib, 1993