From Halopedia, the Halo wiki
The robotic rover is a UNSC self-propelled, multi-function automaton. It has a rudimentary artificial intelligence and responds to basic verbal commands (eg. "stop", "left", "right", etc.) but otherwise does not interact.
Rovers are a cargo carrier with a capacity of several tons.
Dollies are specialized for use in medical facilities. They have arms to retrieve and manipulate items and carry both supplies and patients.
List of appearances