Siva is the Hindu god of destruction. He is one of the gods of the trinity, the others being Vishnu, the god of preservation, and Brahma, the god of creation. Another way to describe Siva is that he represents the destructive aspect of Brahman.
Siva is also considered the god of the yogis and ascetics. He is the embodiment of serenity, renunciation and indifference to the world. He taught the science of Swara yoga to the goddess Parvati in the Siva Svarodaya.
His main mantras are Om Namah Sivaya which means "salutations to Lord Siva", and the Maha-Mrityunjaya Mantra, also known as the Om tryambakam. There are many others as well.
In conjunction with the female aspect of the divine energy, or shakti, in the forms of Parvati, Kali or Durga, he is Siva with form (saguna) for the worship of his devotees. Siva minus the female energy represents the formless (nirguna) aspect of Brahman.
Siva is married to Uma, and his sons are Ganesha and Subramanya.
The trident on Siva's right represents the three gunas - Sattva, Rajas and Tamas. The drum, or damaru, represents the sound of Brahman, or Om. It is Siva who created Sanskrit, the language of the gods, from the sound of the damaru. Sometimes Siva is depicted with four arms, holding the trident in one of his right hands and the damaru in one of his left hands.
The crescent moon on Siva's forehead indicates that he has perfect control of the mind.
From Siva's locks of hair flows the river Ganga. This river represents the nectar of immortality.
He wears the skin of the tiger and sits on a tiger mat, showing that he has conquered lust.
The serpents around Siva's neck denote wisdom and eternity because of their long lives. They also represent the jiva, or individual soul, which rests on Siva, the Supreme Soul. When the individual attains knowledge of the Self through control of the mind and senses, he will find his resting abode in Lord Siva.
He is sometimes called Trilochana, the three-eyed one, because in the center of his forehead is the third eye, the eye of wisdom, or jnana chaksu. This half-lidded third eye also represents his destructive energy that if let loose would destroy the whole world. Half-closed eyes mean that his mind is absorbed in the inner Self while his body is engaged in the world.
The blue markings on Siva's throat represent the poison that he drank from the ocean after a great deluge and battle between the gods (devas) and demons (asuras). He protected humanity by drinking this poison. This gave Siva the name Nilakantha, which means "blue throat".
The lines of ash on Siva's forehead signify that people should try to destroy the impurities of egoism, karma performed with the expectation of fruits, and maya or illusion, as well as the desires for property, husband or wife, and money.
Nandi is his bull. Nandi represents the pranava, or omkara, symbols for Om. He is also the attendant and vehicle for Siva and represents Satsang and the protection of dharma or righteousness. Nandi is tasked with hushing all of nature so that Siva will not be disturbed in Samadhi.
The lingam to Siva's right represents advaita, or non-dualism. It means "I am one without a second." It refers to Siva in the context of the infinite reality.
In the sanctum sanctorum of Siva's temple is an altar, the Balipitha, which signifies that people should destroy their egoism and sense of mineness before they can attain the Lord.
In the background is Mt. Kailas, the abode of Lord Siva.
*Many of the references on this page can be found in more detail in the book, "Lord Siva and His Worship", by Swami Sivananda.