The way ROS uses the 3D model of a robot or its parts, to simulate them or to simply help the developers in their daily work, is by means of the URDF files.

Unified Robot Description Format (URDF) is an XML format that describes a robot, its parts, its joints, dimensions, and so on. Every time you see a 3D robot on ROS, for example, the PR2 (Willow Garage) or the Robonaut (NASA), a URDF file is associated with it. In the next few sections, we will learn how to create this file, and the format for defining different values.