Worship of shivalinga is considered the prime worship for the devotees of Lord shiva. Worship of all other forms is considered secondary. The significance of the shivalinga is that It is the resplendent light (flame) form of the Supreme - solidified to make the worship of It easier. It represents the real nature of God - formless essentially and taking various forms as It wills.
This jyoti swarUpa of God exist in all the shivalinga forms, there are prominent abodes across Indian subcontinent, where It is in a splendid form. These are renowned as dvAdasa jyotir lingas or 12 jyotirlingas. These are held at much great esteem since the ancient pre-historic times. purANas talk in many sections as well as in detail about the glory of these abodes. Devotees have been getting pulled towards these kshetras due to the highly benevolent divine presence in these abodes, since ancient times.