Division Messenger Van is just a big empty van or SUV driven by an aide. Mainly used to just move things around as needed.
Division 15 Messenger Van: http://www.fdnytrucks.com/files/html/brooklyn/e283.htm (bottom picture)
Division messenger vans are also sometimes used to assist with transporting personnel to and from active incident scenes at the change of tours to relieve off going personnel. This was the case at the 3rd alarm in the 13th Division this morning.
Generally, the division messenger van is staffed by a light duty firefighter. When not performing any of the above mentioned duties, he fills the role of messenger which involves picking up and transporting equipment, department paperwork, etc.