I don't know much about marketing, but from my understanding a target demographic is the type of audience you are trying to attract to a TV show, radio program, magazine or other form of commercial media. Target demographics help advertisers know who is going to be watching/listening/reading and therefore what products to advertise based on the expected audience. For example the target demographic for the Pokemon TV show is obviously children age 6-12 (and their parents), therefore most of the commercials during this show are for children's toys. The target demo for Maxim magazine is men age 18-40, so most of the ads feature half-naked women and are for products like alcohol, clothes, and cars.