Glossary
URDF
Unified Robot Description Format — the XML-based file that describes a robot's links, joints, kinematics, and visual / collision meshes.
URDF (Unified Robot Description Format) is the standard way to describe a robot's body in robotics software. It's an XML file that lists every link (rigid body), every joint (connection between links), the kinematics (parent/child, axis, limits), and references to mesh files for visualization and collision.
URDF is what ROS, MoveIt, RViz, Gazebo, Isaac Sim, and most simulators consume to know what your robot looks like and how it moves. Most published robots ship a URDF; if you build a custom robot, writing or generating a URDF is one of the first tasks.
Limitations: URDF can't describe closed kinematic chains (four-bar linkages, parallel mechanisms) cleanly. Extensions like SDF (Simulation Description Format, used by Gazebo) and URDF+ relax this.