A demographic is a specific section of society, defined by certain parameters e.g. race, class, age, gender.
Advertisements work best when they carry a concise or direct message.
This might be a message which distinguishes a brand from its competitors – think how the BMW adverts work, reinforcing the image of their cars as supreme driving machines and conveying the impression of quality. You never see a female model in a BMW advertisement or hear of ‘free finance’. Compare a BMW print advertisement with one for Peugeot which offers cars that are good value for money.
A company has to decide the form of the advertisement. Is it to be a television commercial (very expensive), or a cinema commercial (even more expensive), radio or print or billboard. It will look at the market it wants to reach. It might choose a magazine rather than a newspaper.
E.g. Orange decided that the specific market they wanted to reach was the mainly young people in their twenties who go to the cinema. They created a series of cinema very funny and creative advertisements with Kevin Spacey, which have been enormously successful. They succeeded in promoting brand awareness and introducing the audience to new mobile phone products.