A symbol is a form or mark on one level of consciousness that represents a truth at higher level. For instance, in all Dharmic traditions saffron color represents renunciation and seeking of reality in one of the multiple ways.
Shiva Linga or Lingam represents the creative aspect of God. In Sanskrit, Linga means a mark or a sign. Thus the Shiva Linga means Auspicious ( Shiva) Symbol (Linga) - that reminds us of the infinite qualities of the Lord. Now, creation is not possible unless there is destruction and removal of that which has become useless or will become obstacle to new creation. So, Shiva is also the Lord of destruction and dissolution of old, obsolete and objectionable. Even in this aspect He is still auspicious as in His creative aspect. So, one who is real bhakta of Shiva does not repulse by the destructive aspect of Shiva. Both creation and dissolution require concentrated energy. Therefore, Shiva is the also the Lord of Tapas (concentration of consciousness) and Vairagya (enlightened detachment).
Shiva Linga is physically depicted by a round or cylindrical shape seated on a round flat base. It consists of three parts, the lowest of which is called the ‘Brahma-Pitha,’ the middle one, the ‘Vishnu-Pitha’ and the uppermost one, the ‘Shiva- Pitha.’ The roundness represents the formlessness of the Ultimate Reality. The Linga can be made of ordinary stone, marble, metal, clay, precious stones etc. There is also a constant pouring of water on the top of lingacalled Abhisheka. It can be interpreted in several ways depending on your own level of devotion and consciousness:
A devotee who is a karma Yogi, is pouring out his/her all thoughts, emotions, and actions to beloved Shiva;
to a Gnani, It can mean that all world processes are ultimately dissolved in Shiva, the auspicious and the infinite;
To a Bhakta, it also means, by pouring cool water (sometimes milk), an offering of a prayer that Shiva may show only His blissful and peaceful aspect and not His frightening aspect;
To Yogi, Shiva Linga also represents his/her own soul ( Atma-Linga) and abhishek is of all that is pouring in him/her from outside or from his/her own thoughts, emotions and actions, and absorbed in the Atma-Linga.
Philosophically, it means that infinite and all comprehensive wisdom is pouring down (as symbolized by the Mother Ganga) with tremendous force that only Shiva can receive calmly and then in turn let it out as a trickle for the welfare of the world;
Many divine aspects of Shiva are described as stories in Shiva Mahapurana and in the large body of shaivaite literature. In Ramayana we learn that both Lord Rama and the demon king Ravana worshipped Shiva! Lord Krishna also used to visit Somanath. Arjuna was a Shiva Bhakta.
There are a large number of holy places with Shiva Linga. Of these, twelve are very ancient and very holy: