In the Indian languages, the word upayoga means, 'that which is useful'. Upa Yoga, is sub-yoga or semi-yoga. Yoga is for a committed practitioner, who wants to attain the the ultimate. Upa Yoga is for someone who doesn't yet have the inclinations for that. Upa Yoga is a system of yoga that is oriented more towards the physical, and not strongly towards the spiritual. Upa Yoga is a good for beginners, for people to start yoga.
Yoga means union. Today modern science has proved that the whole existence is one energy. If you break the illusion of separateness, and experience life as one whole - that is yoga. Hatha Yoga uses the body postures to move energy in a certain way, in specific directions, to lead one to experience this wholeness. In Hatha, "ha" means sun, and "ta" means moon, so Hatha yoga is to bring a balance between the sun and moon (or Ida and Pingala, the left and the right) within the system.
In this article Sadhguru dispels the misconceptions around tantra yoga. It is a total misinterpretation in the west, that tantra yoga means uninhibited sex. "Tantra", means technology. It is a technology to go beyond the compulsiveness of the body and mind. Tantra yoga is not to fulfill your sexual needs. Sadhguru says - "Unlike sexuality, which tends to find release at the lower level of the energy system, tantra is about building our energies to the fountain-head of the uppermost dimension of the energy system". Tantra yoga, has two paths, a cruder left-hand tantra, and a more refined, right-hand tantra.
In the yogic culture, kundalini is symbolized by snake. Kundalini is energy within you what has not manifest. If one's kundalini is aroused, miraculous things will happen to them. It will also significantly enhance their perception. Before activating the kundalini, it is very essential that the body and mind have been sufficiently prepared. Kundalini yoga is the most dangerous form of yoga. Pumping up of energy without necessary preparation, and proper guidance, should not be attempted.