It is always a bit tricky to give a proper definition of a pin-up girl. To give the most simple and easiest definition, a pin-up girl is a girl, model or an actress that many a men buy the poster of and pin the posters on to their walls and adore them for a long time. Pin-up girls photography started during the World War II when arguably many soldiers used to have posters of many a seductress photographs of Betty Grable and Rita Hayworth, arguably the hottest pin-up girls ever. Pin-Up girls are many times also referred to as Sex Symbols.
The models in a pin-up photography display style, attitude, innocence, glamorous and cunning look. The models usually have scrumptious hair and voluptuous bods. It is left to the photographers to transform the photographs of these models into a concept of art. The make-up of the girls play a really important role on how the photograph turns out to be. But, even the photographer needs to play elegantly with the colours and the contrast, the reflectors and the light. I found a very good article on How to Photograph a Pinup maybe you could have look at it to try something new in photography.
In the meanwhile you may also want to read some photography related articles