No one exactly knows when the worship of Linga or Lingam started in Hindu religion. But throughout the history the worship of Linga has been identified with Lord Shiva. And thus Linga is also known as Shivling or Shivlingam and is of great importance in Hinduism.
Significance of Shivling

Bansi Pandit writes in Hindu Dharma.. “linga is derived from the two Sanskrit words laya (dissolution) and agaman (recreation).” Thus the linga symbolizes both the creative and destructive power of the Lord and great sanctity is attached to it by the devotees.
Today, the ‘ling’ has numerous interpretations and symbolisms. Thousands of pages have been written about the symbolism of Shivling.