Originally, Asura, in the earliest hymns of the Rig Veda, meant any supernatural spirit—good or bad. Hence even some of the devas (demigods), especially Varuna, have the epithet of Asura. In fact, since the /s/ of the Indic linguistic branch is cognate with the /h/ of the Early Iranian languages, the word Asura, representing a category of celestial beings, became the word Ahura (Mazda), the Supreme God of the monotheistic Zoroastrians. But very soon, among the Indo-Aryans, Asura came to exclusively mean any of a race of anthropomorphic but hideous demons. All words such as Asura, Daitya (lit., sons of the demon-mother "Diti"), Rakshasa (lit. from "harm to be guarded against") are translated into English as demon. These demons are inherently evil and are in a constant battle against the demigods. Hence in Hindu iconography, the gods and demigods are shown to carry weapons to kill the asuras. Unlike Christianity, the demons are not the cause of the evil and unhappiness in present mankind (which occurs on the account of ignorance from recognizing one's true self). In later Puranic mythology, exceptions do occur in the demonic race to produce god-fearing Asuras like Prahalada. Also, many Asuras are said to have been granted boons from one of the members of the Hindu trinity, viz., Brahma, Vishnu and Shiva when the latter had been appeased from penances. All Asuras, unlike the devas, are said to be mortals (though they vehemently wish to become immortal). Many people metaphorically interpret these demons as manifestations of the ignoble passions in human mind.
On the account of the Hindu theory of reincarnation and transmigration of souls according to one's Karma, other kinds of demons can also be enlisted. If a human does extremely horrible and sinful karmas in his life, his soul (Atman) will, upon his death, directly turn into an evil ghostly spirit, many kinds of which are recognized in the later Hindu texts. These demons could be Vetalas, Pisachas, Bhutas etc.