A picture is worth a thousand words:
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.