According to Linga Purana once Brahma and Vishnu started fighting over who was the supreme being. Suddenly, there appeared a Sivalinga before them as a huge column of fire. Both of them decided to find one end each. Whoever returned first would be acknowledged as supreme. Vishnu assumed the form of a boar and went down. Brahma, in the form of a swan, flew upwards. They searched for days but in vain. Then Shiva appeared in the fiery column. Brahma and Vishnu realised their mistake and acknowledged Shiva as the Supreme Being. The Shivalinga represents that column of fire as 12 Jyotirlingas.

There are 12 famous Jyotirlingas where Lord Shiva appeared in the form of light (Jyothi) lingam. These have been worshipped from time immemorial. They are mentioned in the Dwadasa Jyotirlinga Stotra given here.
 

Vaidyanath Jyotirlinga Bhimashankara Kedarnath       Mahakaleswar Mallikarjuna Omkaresswar       Somnath Trayembakeswar Rameshwaram       Nagneswar Kashi Vishwanath Grishneswar