Identity of the Trinity

Siva is one of the deities of the Hindu Trinity. He is responsible for the dissolution of the universe. He is the embodiment of Tamas, the centrifugal inertia, the tendency towards dispersion and annihilation.

Literally, Siva is one in whom the universe ‘sleeps’ after destruction, till the next cycle of creation starts. All that is borne must die. All that is produced must disintegrate and be destroyed. This is an inevitable law. The principle that brings about this disintegration, the power behind destruction, is Siva.

But Siva is much more than this concept of dissolution. Disintegration of the universe ends in the ultimate thinning out, into a boundless void. This boundless void, the substratum of all existence, from which springs out again and again this apparently limitless universe is Siva. So, although Siva is generally described as responsible for destruction, he is equally responsible for creation and existence. In this sense Brahma and Vishnu are also Siva.

We have a reference to the Trimurti conception of Brahma, Vishnu and Siva in IV.5 of the Maitri Upanishad. It says “These are but the chief forms of the supreme, the immortal, the bodiless Brahman. To whichever one each man is devoted here in this world he rejoices for verily this whole world is Brahman”. This Upanishad traces the three forms to the three gunas rajas, sattva and tamas in V.2. It indicates the relation of the three forms (murti-traya) to the Supreme. The three, Brahma, Vishnu and Siva are not to be conceived as independent persons; they are the threefold manifestations of the one supreme.   

The Upanishad further states “Verily, in the beginning this world was Darkness (tamas) alone. That, of course, would be in the Supreme. When impelled by the Supreme, that goes on to differentiation. That form, verily, is Passion (rajas). That Passion, in turn, when impelled, goes on to differentiation. That, verily, is the form of Purity (sattva).

That Purity, when impelled, flowed forth as Essence (rasa). That part is what the intelligence principle in every person is—the knower of the body which has the marks of conception, determination, and self-love, Prajāpati (Lord of Creation) called Visva. These forms of Him have previously been mentioned.
Now then, indeed, the part of Him which is characterized by Darkness (tamas)—that, O ye students of sacred knowledge, is this Rudra. Now then, indeed, the part of Him which is characterized by Passion (rajas)—that, O ye students of sacred knowledge, is this Brahmā. Now then, indeed, the part of Him which is characterized by Purity (sattva)—that, O ye students of sacred knowledge, is this Vishnu.
Verily, that One became threefold. He developed forth eightfold, elevenfold, twelvefold, into unlimited parts. Because He thus developed He is a created being (bhūta); has entered into and moves among created beings; He became the overlord of created beings. That is the Soul (Ātman) within and without—yea, within and without!” (Dr.S.Radhakrishnan)
This identity of the Trinity is also portrayed in several stories in the Puranas.

Forms of Siva
Siva is worshipped both in the form of an image or a picture and as the linga, the latter being the rule particularly in the temples while the former is an exception. The most common features of his pictures or images show him as a very handsome youth, white as camphor. His limbs besmeared with ashes are strong and smooth. He has three eyes - the third eye being on the forehead between the eyebrows - and four arms, two of the arms holding the trishul and damaru while the other two are in the abhaya (protection giving) and varada ( boon giving) poses. He has a crown of long matted hair from which flows the river Ganga. He also wears the crescent moon as a diadem. A tiger skin and an elephant skin adorn his body as his garments. There are serpents all over his body forming the necklace, the girdle, the sacred-thread, and arm bracelets. There is also a garland of skulls round his blue neck.

He is conceived as a deity having his family consisting of his consort, Parvati, sons, Ganesha and Skanda along with a large retinue called ganas and his vehicle, Nandi (the bull). Though he has his headquarters in the Himalayan Mountains, he is fond of roaming the earth, especially the cremation grounds. All these portrayals are in perfect consonance with his nature as the Lord of dissolution.

After Arati we usually recite the following prayer which carries the picture of Siva depicted above:

Karpura gauram karunaa avataaram, sansaar saaram Bhujgendrahaaram|
Sadaa vasantam hridayaarvinde, Bhavam Bhavaani sahitam namaami ||

I bow to that camphor-hued, white complexioned (Lord Siva), who is incarnation of compassion, who is the very essence of (consciousness; the knowing principle) of life (of the embodied soul);  Who wears snakes as garlands, whose eternal abode is in the heart of the devotee, I bow to Him (Lord Siva) and His consort Bhavani (Uma or Paarvati).