Lord Shiva is so often represented as a "lingam". Linga basically means a sign or symbol. So the lingam is essentially a symbol of the shapeless universal consciousness of Lord Shiva. There are a few versions according to the Puranas of why Shiva is worshiped as a lingam and how this happened, of which I will relate one. There was a great sacrificial ceremony that was going to take place many hundreds of years ago. The great sage Narada Muni was invited to it and asked who would receive the effects of the sacrifice. No one could answer, so the sages who were present asked him who should receive it. Narada said that Sri Vishnu, Brahma, and Shiva were all eligible, but they would have to find out which one had the most patience and purity to be the receiver of the sacrifice. So he chose the great sage Brighu to learn the answer. Brighu had many mystic powers and was able to travel to the domain of the demigods. So first he went to see Lord Brahma, but Brahma was preoccupied and did not notice Brighu’s presence. Feeling insulted, Brighu cursed Brahma, “You are so proud of your power of creation, you did not notice my arrival. For this you shall have no temples on earth.” Thus, there are very few temples of Brahma on earth. Next, Brighu went to see Shiva in Kailash, but Shiva also did not notice Brighu’s arrival. Brighu, again feeling offended, cursed Shiva to be worshiped only as a lingam on earth. This is the reason why Lord Shiva is primarily represented and worshiped as a lingam on this planet.