Shivling is considered to be the sign of love between Lord Shiva and his Wife Parvati. It is believed that when a couple worships the Shivling, their love life remains evergreen. A Shiva Lingam has three distinct parts which are considered as portions of Brahma, Vishnu and Shiva. The Lower part represents Brahma, the middle Vishnu and the upper and the most prominent represents Shiva. Thus Shiva Lingam represents all the three powers in one- as the Param Braham or Supreme Shiva.
theencyclopediaofhinduism: “Dattatreya is the divine teacher incarnate who handed over all the tenets of Tripura Rahasya to Parasurama at Mahur, the well-known Shakti center of India. Thereupon, Parasurama composed all the teachings received from Dattatreya in sutra form and formulated the work known as Parsuramakalpasutra.” -The Encyclopedia of Hinduism