Commonly used messages in ROS. Includes messages for actions (actionlib_msgs), diagnostics (diagnostic_msgs), geometric primitives (geometry_msgs), robot navigation (nav_msgs), and common sensors (sensor_msgs), such as laser range finders, cameras, point clouds.
E.g. https://github.com/ros/common_msgs/blob/d4b7d2bdc65c06d11d9e6a020d719874b5ce371a/diagnostic_msgs/mainpage.dox#L9