The Ajna, or Third eye, is not energy per se but a meridian/centre/channel/collective of energy or prana in some Hindu traditions.
The kundalini is a psychospiritual energy which does reside at the base of the spine. It is, as Guru said, to be taken seriously and not flippantly like most modern day (nonsense spouting) witches and new-agers say. The best account I can think of is "Serpent Fire" by Gopi Krishna.
Kundalini can be awakened with the help of a guru, or it can be awakened on it's own. It is not to be rushed, but gently teased and allowed to run it's course naturally, certain yogic practices and magical practices can aid in this. There is no denying it, if you have doubts whether you have experienced it, you have not experienced it. When you do, you will have no doubt of the energy potential within you.
I liken normal bodily energy which is around us and in the sphere of sensation to prana, I liken kundalini to concentrated prana, located in the root chakra/muladhara. It is volatile and can be self destructive if the vessel is not prepared.