URDF

URDF (Unified Robot Description Format) is an XML schema describing a robot's kinematic and dynamic structure: links, joints, mass properties, collision geometry, visual meshes. The de-facto standard for robot description across simulators (Gazebo, Isaac Sim, Mujoco via converters), motion planners, and control software.

Originated at Willow Garage in the early ROS era and maintained by the Open Source Robotics Foundation. Mujoco's native MJCF format competes for newer work — better physics fidelity, easier composition — but URDF's tooling ecosystem keeps it dominant. Most published humanoid platforms ship URDFs alongside their hardware specs; the URDF is often the first thing made public before commercial deployment, since it's the precondition for third-party simulator and motion-planning work.