Shiva (also spelt Siva, has many names) is form of God in Hinduism. He is the third form of God as God as the Destroyer, one of Trimurti (popularly called the "Hindu trinity"). In the trimurti, Shiva is the destroyer, while Brahma and Vishnu are creator and preserver, respectively. However, even though He represents destruction, He is viewed as a positive force (The Destroyer of Evil), since creation follows on from destruction. Very few Hindus believe in a legend that he came from an egg laid by Ammavaru while the substantial majority say He is anadi (without beginning/birth) and ananth (without end/death). That belief of Ammavaru is not held by the substantial majority of Hindus and is merely a local tradition. Worshippers of Shiva are called Shaivaites.
Some of His chief attributes are signified by His hundreds of names, such as Mahabaleshwar (Great God of Strength), Tryambakam (Three-Eyed One, i.e. All-Knowing), Mahakala (Great Time, i.e. Conqueror of Time), Nilkanth (The one with a Blue Throat) etc.
Shiva is the supreme God of Shaivism, one of the two main branches of Hinduism today (the other being Vaishnavism). His abode is called Kailasa. His holy mount (called vahana in Sanskrit) is Nandi, the Bull. His attendant is named Bhadra. He is usually represented by the Shiva linga (or lingam). He is generally represented in Hindu tradition as immersed in deep meditation, on Mount Kailash (Reputed to be the same as the Mount Kailash on the Tibet-India border, near Manasarovar Lake) in the Himalaya, which is supposed to be his abode.
Shiva's consort is Devi, God's energy or God as the Divine Mother who comes in many different forms, one of whom is Kali, the goddess of death. Parvati, a more pacific form of Devi is also popular. Shiva also married Sati, daughter of Daksha, who forbade the marriage. Sati disobeyed her father and Daksha held a Yagna (ritual sacrifice) to Vishnu, but did not invite Shiva. In disgust, Sati sacrificed herself in the same fire Daksha used in his sacrifice. Shiva arrived at the scene, angry at the death of his wife, and killed many of the guests, as well as decapitating Daksha, though he later replaced his head with that of a goat. Shiva created the monster Virabhadra during his quarrel with Daksha, and he was the leader of Shiva's men who came to prevent Daksha from conducting the Yagna. According to legend (Shivpurana, Ramcharitmanas and other Hindu scriptures), this same Sati was reborn in the house of Himalaya (Who is almost certainly the mountain-range personified) and performed a great tapa (Sequence of austerities, culminating in sustained meditation on the object desired, which in this case, was the Lord Shiva.) This tapa caused Shiva to break his Samadhi(State of deep, usually ecstatic meditation) and accept Parvati as his consort.
Shiva gave Parashurama his axe. Shiva's great bow is called Pinaak and thus he's also called Pinaaki.
Shiva and Parvati are the parents of Karttikeya and Ganesha, the elephant-headed God of wisdom. He acquired his head due to the actions of Shiva, who decapitated him because Ganesha refused to allow him to enter the bath while Parvati was bathing. Shiva had to give him the new head to placate his wife. In another version, Parvati showed the child off to Shiva, whose face burned his head to ashes, which Brahma told Shiva to replace with the first head he could find, an elephant. Karttikeya is a six-headed god (Thus called shadaanan, the one with six heads. Sanskrit: shad, six aanan, head.) and was conceived to kill the demon Tarakasura, who had proved invincible against other minor gods.
According to the foundational myth of Kalism, Kali came into existence when Shiva looked into himself; she is his mirror image.
Another version: She had gone out to kill demons but she went on a rampage. To stop her, Shiva went and lay down on the ground in front of her path. When she stepped on him, she looked down and realized that she had just stepped on Shiva. Feeling ashamed, she stuck out her tongue, and the rampage ended.
As Nataraja, Shiva is the Lord of the Dance, and also symbolises the dance of the Universe/Nature, with all its delicately balanced heavenly bodies and natural laws which complement & balance each other. At times, he is also symbolized as doing his great dance of destruction, called Taandav (Pronounced with a soft 't' and a hard 'd'), at the time of pralaya, or dissolution of the universe.
Some Hindus believe Shiva to be just one of many different forms of the universal Atman, or Brahman, a monistic entity to which all things, Shiva and everything else, are identical. Others see him as the one true God from whom all the other deities and principles are emanations, essentially a monotheistic understanding usually related to the bhakti sects of Shaivism.
Although he is defined as a destroyer (Or rather recreator), Shiva, along with Vishnu, is considered the most benevolent God. One of his names is Aashutosh, he who is pleased by small offerings, or, he who gives a lot in return for a little.
Traditionally, unlike Vishnu, Shiva does not have any avatars. However, several persons have been claimed as avatars of him, such as Shankara. Some people consider Hanuman to be an avatar of Shiva.
Nayanars (or Nayanmars), saints from Southern India, were mostly responsible for development of Shiva sect in the Middle Ages.
The important Shaivite sects were Kashmir Shavaites from Northern India, Lingayats and Virasaivas from Southern India. Saiva Siddhanta is a major Shaivite theory developed in Southern India.
Shiva hasnt done any full life avatars, but short stage dramas to help his devoties (particulary nayanmars). This is greatly explained in Thiruvilayadalpuram. Basically all this happened in South India mostly Tamil Nadu. Madurai being the host for most of the short avatars.
Shiva is an aspect of God or Saguna Brahman,(i.e., God with form) whom Hindus pray to. He is the third God of the Trimurti (also called the Hindu Trinity), along with Brahma and Vishnu. Alternatively, He is God in His form as Destroyer of the Universe. Aspects of God such as Shiva or Vishnu are simply personal attributes of the impersonal Nirguna Brahman, God without attributes, the type of God similar in Semitic religions such as Islam or Judaism.(i.e., God without form or without personal characteristics) The term Hindu god should not be equated with Shiva and is confused with Devas. Devas or demigods, are celestial beings similar to angels as discussed in Judaeo-Christian traditions. Devas in Sanskrit literally means "shining beings."Pychaya Suksrinuch