Shiva (Śiva; // listen (help·info) meaning "The Auspicious One"), also known as Parameshwara (the Supreme God), Mahadeva, Mahesh("Great God") or Bholenath ("Simple Lord"), is a popular Hindu deity and considered as the Supreme God within Shaivism, one of the three most influential denominations in Hinduism. Shiva is regarded as one of the primary forms of God, such as one of the five primary forms of God in theSmarta tradition, and "the Destroyer" or "the Transformer" among the Trimurti, the Hindu Trinity of the primary aspects of the divine. Shiva is also regarded as the patron god of yoga and arts.
Shiva is usually worshiped in the aniconic form of Lingam. Shiva of the highest level is limitless, transcendent, unchanging and formless. However, Shiva also has many benevolent and fearsome forms. In benevolent aspects, he is depicted as an omniscientYogi who lives an ascetic life on Mount Kailash, as well as a householder with wife Parvati and His three children, Ganesha,Kartikeya andAsokaSundari or as the Cosmic Dancer.(AshokaSundari daughter of Shiva is not longer known in some places and beliefs because she is not mention in any puranas but in Padma Purana  ,there is relevant proof of Asoka Sundari which is mention about her presence).In fierce aspects, he is often depicted slaying demons. The most recognizable iconographical attributes of the god are a third eye on his forehead, a snake around his neck, the crescent moon adorning and the river Ganga flowing from his matted hair, the trishula as his weapon and the damaru as his instrument.
Shiva as we know him today shares features with the Vedic god Rudra. Historians have also suggested that worship of Shiva existed in pre-Vedic times, but not all historians agree on this.
Courtesy : http://en.wikipedia.org/wiki/Shiva