a form of non-verbal communication in which visible bodily actions communicate particular messages, either in place of speech or together and in parallel with spoken words.

A gesture is defined as:

The act of moving the limbs or body as an expression of thought or emphasis.

Ref: The Free Dictionary

The movement of gestures can be used to interact with technology, using touch or multi-touch popularised by the iPhone, physical movement detection and visual motion capture, used in video game consoles.

Ref: Wikipedia