A picture is worth a thousand words:

Image: Kinect and Java (by Dekuwa and Silveira Neto)
kinect plus java

To put it simple Josceleton is a free open-source Java library providing access to the Microsoft Kinect device. It allows you interact with multiple users by either looking at their raw coordinates of several joints, or simply use the pre-built detectors for certain gestures and positions!

" ... it enables users to control and interact, ... through a natural user interface using gestures ... " -- Wikipedia

Technicaly speaking Josceleton provides a high-level API to communicate with a OSceleton server instance via OSC (Open Sound Control Protocol), which is done by internally using the Java OSC library.

Video: XBox Kinect OSCeleton Showcase (by Tiago Serra)