The Unified Robot Description Format (URDF) is an XML specification to describe a robot. It is capable of describing linkages with tree topologies. Further information can be found in the http://wiki.ros.org/urdf/XML/model web page
The format describes the following items:
Kinematic and dynamic structure of the robot ( Kinematic graph )
Visual representation of the robot (mesh files/ graphics primitives)
Collision model of the robot (mesh files)
LoadURDFLinkage function imports the kinematic structure and visual representation of the linkage.
URDF file usually reference other files that is stored in a specified directory structure see below:
LoadURDFLinkage uses the full path to the URDF file as input and assumes that the this file is in the /urdf subdirectory of the urdf-root directory ( MYROBOT_description dir ). It is also assumed that the referenced mesh files path is specified in the URDF file relative to the urdf-root directory.