Power paint rollers
Power paint rollers are fed from a pump, usually an airless sprayer and save the time taken with a hand paint roller to coat the roller cover with paint, which is about 50% of rolling time with a hand roller. Because they are fed from a pump and controlled by the painter, they will carry a much more uniform coat of paint, leading to a more uniform coverage of the surface, in turn usually leading to a saving of paint. As the nap of the roller cover does not have to carry the paint, a much shorter nap can be used, leading to a better finish.
Internally-fed power rollers
These power paint rollers have perforated covers with the paint fed inside the cover and distributed along the inside length, before coming out through the holes distributed over its entire surface. The roller head is usually mounted on a heavy duty extension pole which is airless pressure rated so that it can be safely connected to an airless paint pump and the flow controlled by an airless gun at the other end of the pole, usually of the in-line type which is much more comfortable than a standard airless gun would be. You should be careful not to connect a telescopic roller pole that has a low pressure rating and designed for a low pressure system. The roller covers usually have the American 1.5" internal diameter, and are available in several nap lengths to suit the work. Some have a foam layer between the core and the nap to better handle very rough surfaces