I am confused by this question. The phrase "or just running media through my PC to a TV, like movies" makes it sound like you want to know what a HTPC does differently than a PC?
If that is the question you are asking then the answer is simple. Most people have their computer at a desk with a monitor. Usually their main TV is in a separate room. So rather than find the worlds longest HDMI cable they build a small, quite, inexpensive PC that can play some games and stream content to their TV. But it is essentially just a normal PC, with a more specialized function.
Sorry, did not mean to be an ass. Was actually confused by the phrasing.
Yeah an HTPC is simply a PC that you hook up to your home theater. It only has a special designation because it is usually quieter and smaller than a normal desktop, but besides that it is simple a computer.