Mainform 4 :

His supreme creative powers are celebrated in the worship of the Lingam or phallus just as His consorts are worshipped through the female genital or Yoni, which is the source of female energy and always associated with Him.
The lingam is one of the oldest forms in which Shiva is worshipped.
It is also the most worshipped form of Shiva.

The old lingams were erected phalluses but during time these symbols got a more
smooth surface and a top as short pillar. The lingams are often depicted with a Yoni. The Lingam often rest in a yoni witch symbolizes the merging of the male and female, the Creation and the source of Life. The yoni symbolizes the vagina or the life-bringing womb.
It is believed that the people of the Indus-valley already worshipped the Lingam.
Siva is worshiped in the form of the Siva-lingam, or phallus, in most temples. Most Siva temples have a trident on top of the temple tower.
He is known as the “giver” god

Stories about the lingam:

In one story, the phallus of Shiva is cut down. But the phallus started to grow and to grow. The growth symbolizes the immeasurable power of Shiva.

Brahma and Vishnu had a quarrel about who was a higher and more important god.
During this quarrel a fiery lingam appeared between them. Brahma took the shape of a swan and wanted to find the top of the lingam. Vishnu became a boar and started digging.
He wanted to find the base of the lingam. Both came back without success.
Then Shiva revealed himself as Mahasvara (the supreme god). A god without a beginning and end.