The roots of tantra go way, way back and no one really knows the origin of tantra. However, the real blossoming of tantra happened in Kashmir around the first millenia AD. This school is known as Kashmir Shaivism or Shakti-
The word tantra is derived from the root "tan" which means to extend or expand. Tantra is about expanding your conscious awareness of who you are to incorporate the Absolute Self and finally merge with the Self. Once you have realized utter and complete oneness with the Self, you are knows as a jivan mukta, a person who is liberated while still living in this body and personality.
Another meaning of the word is this: A tantra is a scripture belonging to a Shaiva or Shakta (see the page about Shiva and Shakti) tradition. Though I will here deal with Hindu Tantra, there is also Buddhist Tantra, which I know little of. The tantras often take the form of dialogues between Shakti and Shiva. Shakti comes in many incarnations or forms: Devi, Bhairavi, Parvati, and so on. The tantras were largely destryed by the Muslim invaders into northern India, where they murdered the tantric teachers and burned their texts. We know of numerous tantras that have been lost, since there are references to them in the texts that have survived.
Is Tantra about sex?
Actually no! Out of the about 200 tantras that have survived to this day, only one (1) in passing mentions sex. So please understand this: What is called Tantra today and which mainly deals with sensuality, sex and extended orgasms has nothing to do with true Tantra. How this sad confusion came about, I have no idea. But if you today say you are a tantric teacher, as I am, everybody at once thinks about sex. Here is the verse that mentions sex, it is from Abhinavaguptas Anuttarastika, verse 4:
Unlike the pleasure of attaining wealth, taking wine, enjoying sex, contact with a young lady, unlike the light of a lamp, the sun, the moon, unlike the joy of getting rid of a heavy weight, the delight of experience of oneness with the realization of the illumination of consciousness is like recovery of the forgotten precious wealth.
Is the bliss of an extended orgasm the same as the bliss of the Self?
No! It is not! Having been a tantric practitioner for 40 years I have of course tried various sexual so called tantric practices and do know what it is like to have a prolonged orgasm that last like half an hour or more. But since I am also a tantric adept who is a jivan mukta and enlightened, I can ensure you from direct experience that the bliss of sex is not the bliss of the Self.