What Does "Activated" Mean?

So, what is "activated" carbon, and is there such a thing as non-activated carbon? The heat used in "activating" the carbon drives the impurities out of the carbon itself, leaving places for the impurities of the water or air or whatever it is the carbon is filtering to reside. In a way, carbon can act sort of like a sponge, only instead of squeezing out its contents, one heats it out. Once the activated carbon is full, it can be reactivated by re-heating it, though this is considered to be impractical and dangerous, especially in the case of a HEPA air purifier where the carbon is going to be a very, very small pat of the filter, and probably encased in plastic. Non-activated carbon would basically be carbon that has stuff in it already, such as soot or charcoal.