Kinect from Microsoft is a motion, gesture and sound sensing system that uses a proprietary stereoscopic webcam and microphone to sense physical position, movement and voice commands from the user. While it was originally developed for the XBox 360 gaming console system, it is now a popular and supported peripheral for Windows PCs as well.
Kinect has a Windows SDK, that permits high-level code to accept gesture and movement input. Quoting from the Kinech API page:
Kinect for Windows gives computers eyes, ears, and the capacity to use them. With Kinect for Windows, thousands of businesses and developers are creating applications that put people first—allowing their customers to interact naturally with computers by simply gesturing and speaking.
For people familiar with Windows / .net software development, the Kinect SDK is known to be very easy to use. A Kinect device will be required for Kinect development, a generic webcam isn't good enough.