Basically, there are two types of Buffer or Polisher you can get. Your one, sounds like an 'Orbital Type', meaning that the pad spins completely still, just on one spot. The Orbital Types tend to do a more dramitic job, but because it has a tendancy to stay over one patch of paint, it heats the paint quickly and without proper training, you can easily burn the paint.
A PC is basically a brand-name, called Porter-Cable. People refering to a PC here are talking about their product known as a "Random Orbit Polisher". The Random type not only spins on one axis, but also the buffer pad moves around in a circle like fashion, while spinning in the meantime. This means, that it has less chance of burning the paint, while still giving a similar result. You can get the same results, with a little more effort.