Shiva (also spelt Siva) is a Hindu god. He is the third god of the Trimurti (popularly called the "Hindu trinity"). In the trimurti, Shiva is the destroyer, while Brahama and Vishnu are creator and preserver, respectively. However, even though he represents destruction, he is viewed as a positive force, since creation follows on from destruction.
Shiva's consort is known as his Shakti. Shakti comes in the form of several goddess, one of whom is Kali, the goddess of death. According to the foundational myth of Kalism, Kali came into existence when Shiva looked into himself; she is his mirror image. Kali is often depicted having sexual intercourse with Shiva's dead and lifeless body. Shiva is dead to signify his passiveness in relation to Kali.
Hindus believe Shiva to be just one of many different forms of the universal Atman, a monistic entity to which all things, Shiva and everything else, are identical.
Traditionally, unlike Vishnu, Shiva does not have any avatars. However, several persons have been claimed as avatars of him, such as Shankara. Need more detail on 'official' (if there is ever such a thing in Hinduism) view on this...
What are the major Shaivite texts/schools?