Shivaling literally signifies the body part of Lord Shiva, and only after the word "OM", it is the next most important and powerful word for the Hindus. Interestingly enough, in almost all the Hindu temples across the length and breadth of India, it is the Shivaling only which is worshipped, and it is only in the rarest of rare cases that you will find the images of Lord Shiva in these temples !
A Shivaling is an object with a round or cylindrical shape , which is fixed in a horizontal position , held firmly in place by a round circular base . It has a close resemblance to a male sexual organ, which signifies the creative power of Lord Shiva , and the circular base has a close resemblance to the female sexual organ, signifying Lord Shiva's consort Godess Parvati.
In actual fact , the physical aspect of the shivaling symbolises the male and female organs in a state of conjugal bliss, and on the mental plane it symbolises the union of the mind and body. The Shivaling is also symbolic of the supreme self, the highest self , and the Lord oof the Universe. It has three parts, the lower part representing Lord Brahma , the middle part being octagonal in shape and representing Lord Vishnu, and the upper part which is cylindrical in shape is representative of Rudra. This part is in fact the prayer portion which receives all the puja offerings like milk, flowers, fruits etc .
While you will usually find the Shivalings in the temples, on some occasions ardent worshippers also install them in their homes, and in these circumstances it is very important to ensure that you offer prayers to the Shivaling on a regular basis, since it is considered to be a source of divine energy. Shivalings have been found to have been created at various places, purely by forces of nature, and these natural formations are considered to be a very good omen, with temples having been created at such locations .
There are twelve such significant locations in India where the Shivalings were found in a state of natural existence, called as jyotirlings, and Lord Shiva is considered to be swyambhu, i.e. present at these places. Temples had been built at all these several hundred years ago.
On the other hand , as compared to the Shivalings created by the natural forces, you also have the artificial Shivalings which are are crafted out of various materials including glass, wood, clay, gold, crystal, diamonds, precious stones etc. Sometimes they are made of sandalwood or clay for the purpose of temporary worship, and after the worship is over they are disposed off. You will also find that there are several people who resort to wearing of the shivalings on their body or around the neck, since they consider it to be a source of tremendous energy !