This gesture is widely used - not always in a religious context.

Certainly in India, the birth place of the Buddha, it is a common gesture of greeting, respect and reverence and is usually offered between equals, or from junior to senior. As the Buddha is the most senior of all, it is rarely found on a Buddha statue.

The symbology is manifold.
It is a symbol of unity where the two opposites: the left and the right, the black and white, yin and yang, male and female, etc - i.e. the whole dichotomous universe - comes together as one. This is the idea behind this web site: the Buddha Mind.
This union takes occurs over the heart centre - the place of emotion, of love, etc. There is much written in yogic texts about this cakra.
The hands form the shape of a lotus bud - a potent image in itself, rich with symbolism.
The hands can also be seen as forming a diamond (shape). With the two 'worlds' joined the resulting union is seen as symbolic of the eternal, indestructible truth that is the Dhamma.
The intelocking ten fingers represent the ten paramittas - the virtues [see: TEACHINGS - NUMBER GROUPS]