You want to know the difference between an Art Model and a Pin-up Girl? Both are female, both are attractive, and both are posing. But the Art Model looks away from you or doesn't focus on anything, while the Pin-up Girl looks right at you. The Art Model is pretty and non-engaging, while the Pin-up Girl's have-fun expression invites you to come play with her. The Art Model usually looks like she hates her job, but the Pin-up Girl is portrayed as enjoying life as well as modeling.
The orthodox fine-art world has, until the rise of postmodernism, avoided the pin-up situation while posing their models in a standard repertoire of positions. One of the first artists to break this rule was Edouard Manet, whose nude "Olympia" (1863) shamelessly looked out at the viewer, like the prostitute she was. Pin-up girls' images were used to make men feel good, but the pin-up image was not necessarily meant to involve prostitution. She could be clean and sexy at the same time.
Both Art Models and pin-up girls are predominantly female, though there are plenty of male pin-ups aimed at the Gay market and sometimes for women. I have always wondered why there are so few male art models. They are probably uncomfortable posing in briefs or even more, nude in front of a mixed crowd.
Nowadays there are plenty of picture books with both female and male Art Models and I have most of them. This sketch at the top comes from one in the "Virtual Pose" series. As with classic models, they turn their faces away from the camera. One of the nicer things about these model books is that they have included different ethnic, racial, and body types among both their men and women, so you aren't just drawing pallid young girls.
I drew this one directly on the Cintiq, it is my first try at a virtual life drawing on it.
Photoshop on Cintiq, about twenty minutes, January 21, 2016.