Disclaimer: This article is based on my understanding made through reading various religious and spiritual books. Being a science student I have always looked for reasons behind everything, even when I know that some things can not be explained. Still, I have tried. Fool me. In no way my intentions are to hurt feelings or beliefs of anyone. In fact I am not strong enough to hurt your beliefs. So, try to read the below article with an open mind, otherwise you always have the choice of not reading it.
Every time we bring Lord Shiva to our thoughts, there are generally two images that come to our mind. One that has form of a male human God wearing animal skin, having a drum (dumroo) in the hand, Ganga flowing throw hairs and snakes curled around the blue colored neck. Another image is that of a Shiva Linga, to be precise, denoting male phallus and female sexual organ (yoni) at the base.
But, why these two representation of Lord Shiva?
First lets discuss that why we are praying to what looks like the sexual organs of male and female body. So, while I was reading about the Kundalini, the Seven Chakras, and spiritual course of universe coming into existence from a single energy source. It struck me that what I have read in various religious books like Vedas, Shuk-Sagar and Shiv Puraan about Devas location and their role in universe creation, is actually so much related to our body, the Chakras inside and the sleeping Kundalini residing in the body.
Kundalini which dwells in the base of our spine in the Muladhaar Chakra, sleeps there in the coiled form. It rests in coiled form and thus sometimes expressed as a serpent, also named Bhujungi. This Kundalini when awakened is The Shakti, the ultimate energy form in its full power. Shakti which as per Indian Vedas and Puraans is the AadiShakti and companion of AadiShiva. Shiva who is static, used his Shakti the kinetic energy, to create the universe and everything in the universe. It is The Shakti, who has created everything as per the intention of Shiva, and she is thus the Mother of all. Sometimes also, referred as Mother Nature(Prakriti), and Maya the illusion.
Shakti and Shiva both complement and complete each other. Without his Shakti, Shiva is nothing, and without Shiva, Shakti is of no use. This can be stated in simple terms as, if Shakti is power, than Shiva is the Power Holder. If the power holder is not able enough or deserving to hold the power, then the power can not be put into any use. Same way, if a deserving power holder do not have enough power in his hands, he can not do anything useful. Thus, a union of Shiva and Shakti can be the only one powerful enough to create and hold everything in the union as one. That’s why when Shiva created this universe (Srishti), with the help of his Shakti, he took the form of Ardhnarishvar, God represented as half male and half female form.
Coming to how Kundalini is related to the universe creation and Shiva Shakti. Not going into much details here, (maybe in some another post), When Shakti was creating the Universe, one form that she took was, of Prakrti. As Prakrti she evolves Mind (Sixth Chakra), senses and matter of five forms namely Ether (fifth Chakra), Air (Fourth), Fire (Third), Water (Second), and Earth (last Muladhaar Chakra). When Shakti entered the last Chakra, her last creation of universe, she then went to rest there in her coiled sleeping form. This sleeping coiled Shakti is termed as Kundalini. This coiled Shakti, in her lowest form is represented as a serpent (Shakti) clinging to Linga (Shiva),or a yoni (Shakti) making a base for Linga (Shiva). Thus, when we are praying to a Shiva Linga we are also praying for the awakening of the Shiva’s Shakti.
In her true or active form, this Shakti travels through all the six Chakras upwards and move to the seventh Chakra, Shashrara lotus Chakra or Crown Chakra, the Chakra of Shiva. Shiva is said to reside in the Crown Chakra, the highest point. Shakti or Kundalini when awakened to her highest state moves to the Crown Chakra to unite with her Shiva. Here, with the union of Shiva and her Shakti, Moksha is achieved, as this is the stage of God. Here, Shiva and Shakti in union are represented as a point (Bindu), which is in reality the point of creation and also destruction of universe.
Now, to mention why Shiva is represented as two forms, one as Shiva Linga and other as a God in human form. My rendition is that, Shiva when represented as Shiva Linga is shown in his true formless nature, the Aadideva Shiva, in union with his Shakti. But, Shiva when represented as human god, is Mahadev (not Aadideva), along with Parvati by his side (but, not in union). This is the place which Mahadev or Rudra has taken along with other MahaDevs like, Vishnu and Brahma to take care of the Srashti (Universe), created by the Aadidevas Shiva-Shakti.
Hope my interpretation is helpful in explaining what can not be explained.