What is kundalini?
"Kundalini"' literally means coiling, like a snake. In the classical literature of hatha yoga kundalini is described as a coiled serpent at the base of the spine. The image of coiling, like a spring, conveys the sense of untapped potential energy. Perhaps more meaningfully kundalini can be described as a great reservoir of creative energy at the base of the spine.
It's not useful to sit with our consciousness fixed in our head and think of kundalini as a foreign force running up and down our spine. Unfortunately the serpent image may serve to accentuate this alien nature of the image. It's more useful to think of kundalini energy as the very foundation of our consciousness so that when kundalini moves through our bodies our consciousness necessarily changes with it.
The concept of kundalini can also be examined from a strictly psychological perspective. From this perspective kundalini can be thought of as a rich source of psychic or libidinous energy in our unconscious.
In the classical literature of Kashmir Shaivism kundalini is described in three different manifestions. The first of these is as the universal energy or para-kundalini. The second of these is as the energizing function of the body-mind complex or prana-kundalini. The third of these is as consciousness or shakti-kundalini which simultaneously subsumes and intermediates between these two. Ultimately these three forms are the same but understanding these three different forms will help to understand the differerent manifestations of kundalini.