The frame of the cosmic reality, according to ancient Hindu thought, consists of the three fundamental states called evolution (Shrishthi), existence (sthiti), and involution (samhara) that acts in a cyclic process of infinity. Each one of the forms is controlled by a God, named Brahma (the creator), Vishnu (the preserver) and Siva/Shiva (the destroyer); these three Gods are called the Trinity. Shiva, being the last to complete the cycle from where the new cycle starts, is known as Mahadeo, the Supreme Divinity. The iconographic form of the Shiva, the Linga represents the unity of the three states of cosmos (shown in above figure).The Linga consists of the three parts. The first is a square base of three-layers at the bottom showing the three mythical realms (lokas), symbolizing evolution the place of Brahma.
As regards the Linga, the emblem of Siva universally venerated, some explanation is needed. Literally Siva means auspiciousness and Linga means a sign or symbol. Hence the 'Sivalinga' is just a symbol of the great God of the universe ('Mahadeva') who is all-auspiciousness. As already explained 'Siva' means the one in whom the whole creation sleeps after dissolution. 'Linga' also means the same thing-a place where created objects get dissolved during the disintegration of the created universe. Since, according to Hinduism, it is the same God that creates, sustains and destroys the universe, the Sivalinga represents symbolically God Himself.
Whether the Sivalinga is a phallic emblem or not, is a moot point. Phallic cults have existed in all countries and in all civilizations. It is quite likely that the phallic cults of an aboringinal civilization were absorbed into Hinduism and the worship itself was elevated to honour the Father Mother-Principle of creation. This is one view. That it is a remnant of the Vedic Yupastambha, to which sacrificial victims used to be tied, is another view. According to this view, the Hindu temple is a metamorphosis of the Vedic Yagasala (sacrificial shed). That it is an imitation of the Buddhist stupa is another guess that is sometimes hazarded but not subtantiated, since Sivalingas have been found even in the pre-Buddhistic civilizations of Harappa and Mohenjo Daro.
Since God is beyond name and form, and since we cannot conceive of an abstract principle like Him, without the aid of concrete symbols, a rounded surface is perhaps the nearest approach to him.
Sivalingas may be Cala (movable) or Acala (immovable). The Cala-Lingas may be kept in the shrine of one's own home for worship or prepared temporarily with materials like clay or dough etc., for worship and dispensed with after the worship or worn on the body as Istalinga as the Virasaivas do. The Acala-lingas are those installed in temples. They are usually made of stone and have three parts. The lowest part which is square, is called Brahmabhaga and represents Brahma, the creator. The middle part which is octagonal, is called Visnu-bhaga and represents Visnu, the sustainer. These two parts are embedded inside the pedestal. The Rudrabhaga which is cylindrical and projects outside the pedestal is the one to which worship is offered. Hence it is called Pujabhaga.
The Pujabhaga also contains certain lines technically called Brahmasutra, without which the Linga becomes unfit for worship.
** Special Note:
Above all information regarding Shivlinga and it's description has been taken from "The Linga Puran" and the book "Banaras Region: A spiritual & Cultural Guide. Indica Books, Varanasi. Piligrimage & Cosmology Series: 1 written by "Prof. Rana P. B. Singh & Dr. Pravin S. Rana