This package holds the diagnostic messages which provide the standardized interface for the diagnostic and runtime monitoring systems in ROS. These messages are currently used by the diagnostics Stack, which provides libraries for simple ways to set and access the messages, as well as automated ways to process the diagnostic data. These messages are used for long term logging and will not be changed unless there is a very important reason.
diagnostic_msgs is a package to hold the diagnostic messages
These messages are used by the runtime diagnostics system.
See the wiki at http://wiki.ros.org/diagnostic_msgs for a summary of the messages.
And the diagnostic system is documented at http://wiki.ros.org/Diagnostics