A woman can be beautiful, perhaps even close to physical perfection, but still have something about her that makes her not attractive or attainable (obviously an unattainable woman can't be considered "ideal"). Conversely, another woman can be obviously not beautiful in the conventional sense but have men buzzing around her like flies. In most cases this comes down to one thing: sexuality or sex appeal. If taken literally, sex appeal is (in this case) a woman's physical quality that encourages men to want to have sex with her. In contemporary usage it implies something a little more subtle, although ultimately the end result may still be the same. Unlike beauty, sexuality can't be measured using a mathematical equation, and probably for most people it doesn't need to be, although it can be monitored and measured in terms of stimuli, physical responses and chemical reactions.
Sexiness can be subtle and even subliminal, but usually it is blatant and overt, relying on simple tricks that have stood the test of time. Take the stereotypical image of a lady prostitute. She is not dressed in baggy pants, sneakers and a sweat shirt, because generally speaking these do not "turn men on". Instead this image sees her dressed in a short skirt, high heels, perhaps fishnet stockings, and more often than not red lipstick and thick black mascara. It's a classical stereotypical image because it (or variations of it) has proved to be successful in attracting men for countless years. However, sexuality in everyday terms usually isn't so easily classified. A lady can appear "sexy" simply by how she sits or walks, how she looks at you, even the way she speaks, but whether intended or not, they still give the impression that she is sexually ready and available. As for clothing and makeup, while the formerly described baggy pants and sweatshirt may be considered generally sexy by a small minority, most people find short or tight skirts, high heels, tight tops, bright makeup, long hair and the presence of the colour red more sexually appealing. This is primarily because they all make a woman look healthier and fitter to our subconscious minds, which goes back to the idea that we are driven by our DNA to seek out the healthiest and fittest partner to continue our genes into the next generation. Tight jeans and shorts also score highly, probably because they present both an idea of a woman's shape underneath, together with the enticement of what can't be seen. Lingerie has a similar effect.
However, such apparel as described above has only been around a relatively short time. Before this a lady wanting to look sexually alluring might have worn a long dress with tight bodice and billowing skirt, with the emphasis on her bust and backside. Indeed the long dress is still with us, and is considered the clothing of choice for weddings, prom nights, evening functions, and so on. While the aim of encouraging a man to have sex is probably not the case, these dresses make a clear point that the wearer is most definitely a lady, both elegant and feminine.