a dolly is a platform of sorts with wheels or castors
a handtruck is a fulcrum based cart used to move odd, bulky or weird shaped items with leverage.
The two often get confused.
A dolly, as defined above, was often used in old films to steady the camera as it moved for cutting edge action shots. Imagine that as you will. the camera man held the camera shooting the scene and another man gripped the dolly moving it about. It became an artform and an important job in the film business. And thus the title of Dolly Grip was invented.
I believe the original version of Dolly began in the train industry as a thin platform that moved along the railroad tracks independant of the train. And you know how those Engineer unions are, they make up words just to use them :P