The package contains all the urdf file of the robot and the configuration files. It is the launcher of all the nodes as well but it is not a node itself.
This package takes the information from the motors and computes the odometry based on geometrical data from the robot. It also publishes an odometry frame that is used for gmapping computation.
The feedbacks are the wheels velocity.
The odometry contains the information about speed and position.
This package transforms the information from velocity command of the robot to velocity command for each wheel. It also transforms it from m/s to ticks/s.
This package is used to simulate the motors-wheels part of the robot. This is not used anymore since Gazebo provide a good enough plugin for this.
This package used to launch the simulation, but since all the files are in the description package now this package is only used to store the models and the worlds.