Asuras (who come from the Hindu religion) are the powerful beings who are opposed to the devas. In early Vedic times, they were originally another class of gods. Such important Vedic gods as Varuna and Mitra were classified as asuras. By the end of the Vedic period, however, the asuras had attained their more demonic role.
The Asura concept also bled into Buddhism, where they filled a similar role as they did in Hinduism. They were known as incredibly boastful creatures that were very angry. Being reincarnated as an Asura is thought to be one of the worst possible rebirths.
The Asura initially lived on Mount Sumeru with the other gods until they drank too much Gandapana wine and were summarily exiled to live at the foot of Sumeru. Angered by their banishment the Asura began waging war on the other gods.
Relevance to Ranma ½
Rouge fell into the Spring of Drowned Asura and quickly took on the angry, wrathful persona of the Asura.