The word "Tantra" has many definitions. Scholars do their best to guess, based on language... Some claim it comes from the Sanskrit or Hindi word for fabric or tapestry, meaning that it is woven into one's life. Others say that is comes from two Sanskrit words tanoti and trayati which mean to expand & liberate consciousness. These are likely close translations, the best we can do after all these years...
It is a practice of sexuality that dates back thousands of years, to India and Tibet, to the Far East, Polynesia, and indigenous cultures of all parts of the world. The North America's native Cherokee culture, even practiced Quadoshka, whish seems to be Tantric as it was used to achieve higher consciousness and union with Divinity.
And that is what Tantra is all about: The combination of love and meditation, to make a connection with other planes of existence. Tantra is not a religious philosophy, but it does involve a spiritual component. Those who do not believe in 'other planes' or 'higher consciousness' really can't be involved in the practice of Tantric unions...
Tantra is a way to blend sexuality with spirituality, but not in a mechanical way, with positions & placements, but with connections. Unlike other forms of spirituality, Tantra does not suppress or deny sexuality. Rather, is the only spiritual path that says that sex is sacred. God/Spirit intended for sex to be, and not just for physical pro-creation of more bodies, but for the further expansion of souls.
Sex is sacred and should be approached from your heart and body.
The view of Tantric philosophy is that the body is the visible house of invisible energy. Both the body in form is energy, as well as the soul or invisible energy that lies within it. And the body can be awakened if we let go, release ourselves, to sexual energy.
By viewing yourself and others in this way, you release yourself not only to the pleasures of the body, but to the mind as well. Once you view bodies and souls as energy that is part of the 'one' you release yourself from many things.
You no longer feel the need to control others, in relationships of all kinds. You are less jealous, insecure of partners, and you discover that the real relationship you seek, the soul mate, is already within you. And once you discover that, you let go of insecurities, neurotic behaviors with others, including co-workers, bosses, children etc. This is because when you discover your connections to One/God/Spirit, you see that all others are part of it as well... And therefore part of you as well. Union with Divinity releases you from negative feelings.
It may seem contradictory, but in using sex and the pleasures of the body with spirit, you actually come to be aware to the fact that the body, that materials, are not even real. They are simply layers that need to be looked past, or through, in order to really experience the other layers, or levels.
There is no specific religious practice that one must believe to practice Tantra. In fact, the ultimate goal is to let go of all belief systems. (Once you see this physical layer is not real, how can belief systems be real?) Any teacher or guide that asks you to 'convert' or join is not to be trusted.
Some further exploration on the net:
Definitions
Asiatica Association -- A scholarly look at Tantra.
The Church of Tantra -- A comprehensive site!