Once the Absolute unmanifest Self desides to go through a number of incarnations it separates Shakti out of itself and this separate shakti is known as Kundalini. The first point of contractionof the unmanifest absolute into an ignorant individual is called Bindu and this then manifests as kundalini. Kundalini is thus the primal energy in man.
Once kundalini has separated from the Absolute Self it manifests you as an ignoirant individual. Meaning it manifests your nervous system, your thoughts, feelings, etc. It also is the energy behind sneezing, digestion and all thinking processes. When kundalini has manifested You, it goes "to sleep" slightly above the root chakra, in the perineum, in a place kalled "kanda", meaning "bulb". From here kundalini keeps you alive in every concievable way. This means kundalini within itself has a blueprint of your structure of ignorance with which you currently identify. This also means within itself holds the knowledge to undo this ignorance. Kundalini has to be in special state to undo your ignorance and grant yu enlightenment, and this state is called "awakened".
So, kundalini is a form of Shakti. Once kundalini becomes active in you, you may think it is a foreign energy working in you, but you really have to understand it is your own Self trying to pull you to Self-
This is the normal state of kundalini in your usual ignorant person. Which currently menas 99.999% of the population. I don't know the correct fraction of the population which has moved beyond the sleeping kundalini, but I think one out of 100.000 is being very generous. You should probably divide it with a factor 10 or 100.
The aroused kundalini
This is often confused with a kundalini awakening, but it is not. A kundalini arousal simply means kundalini has ejected an unusual amount of energy into your system and this may be unpleasant (but not necessarily so). What you have to do in this case is to relax into the energy and let it do what it wants to do. Fighting it only makesthings much worse.
The awakened kundalini
This most desirable state of kundalini is best reached through grace. Someone has to give it to you. And this giving of grace is known as shaktipat (see the page devoted to shaktipat). You will find more and more who claim to give shaktipat, but shaktipat is solely the transmission of grace from the enlightened teacher (guru) to the student, that awakens kundalini in the student. Simple energy transmission is NOT shaktipat.
An awakened kundalini is very, very rare. The kundalini seldom awakens by itself and doing practices, like kundalini yoga, to "awaken" kundalini very, very rarely have the desired result of actually aweakening kundalini. They can at best induce kundalini arousals, but not the rare awakening, which lasts for several incarnations pulling you towards sadhana (spiritual practice) over and over again until you are liberated.