There are three differeces between ballpoint and roller ball. As Fred described, the ink for rollerballs is less viscous. This can present a problem for left-handed writers as the ink stays wet a few seconds longer. The second difference is in the ball. The porous surfaces differ since they are delivering different viscosities.
What's the third difference? Marketing. Pen manufacturers have a need to try to differentiate products within their market. Pentel has used low viscosity inks in it's product line for at least 15 years, but did not give it a name until others chose to duplicate the ink and delivery system.
By the way, gel ink falls into the same category. Higher viscosity and a slight change in the ball to deliver the ink. In the 50's, all ballpoints delivered what the manufacturers are now calling "gel" ink.