I may be not 100% technically correct, but the PC (Porter-Cable) is a type (brand really) of dual-action or orbital polisher and there are also other brands/types.
BTW, I found this definition of orbital on another site, it kind of defines why an orbital polisher is sometimes called a dual-action polisher:
The wheel of a random-orbital buffer, on the other hand, operates with two separate motions: a slow circular motion combined with an orbital motion. That is, as the wheel spins, it also orbits slightly off-center around a central point (sort of like the Earth spins as it revolves around the sun). This action essentially imitates hand motion . . .
Hope this helps.