Simply put... they don't give me enough characters to fully answer this question, but I'll lay down what I can for ya... After reading some of these idiotic replies, I just had to chime in. First of all, it isn't a dumb question. In fact, I would wager every person on the planet has pondered this question at least once in their life... Contrary to popular belief, beauty is not based on individual preference, (although it definitely plays a part) but rather on an "ideal" look that all humans find attractive.. and that look centers, first and foremost, around symmetry- the more symmetrical your face, even without well defined cheekbones or other preferred features, the more attractive people will generally find you. However, certain health indicators such as clear skin and eyes also become important when determining facial beauty. However, beauty, as we know, especially in women doesn't stop at the face. Although there is some debate on body type, Men typically prefer women with a slightly narrower waist than hips, flat stomach, a firm but proportionate butt as well as full breasts. Without generalizing it too much: We like the hourglass figure. The reason we prefer these traits is because they are all indicators of fertility and genes we'd like our (female) offspring to possess, so that in turn, they would have a good chance to procreate as well. We want you to have those things if we are going to sleep with you, because at the end of the day, it all comes back to procreation... (subconsciously anyway)
This is just a quick snapshot of what men prefer in women. It just glasses over the surface. There are scores of books and documentaries describing the golden ratio of beauty, how we decide to choose our mates, and why we find certain characteristics attractive and others not... I suggest you read/watch them and make the best of the situation. Beauty is a science that has been finely tuned after thousands of years of mating... No matter the culture or region, it's been proven we all have a very similar concept of what is beautiful and what isn't. However, do not fret, as always, personality plays a large part in what we find attractive... And if you are lucky enough to be both beautiful and have a wonderful/ confident personality: Congratulations... on winning the genetic jackpot... You're life will probably be easier than 90% of the population.
Hopes this helps your question, I didn't include any telltale signs of unattractiveness, but you can just take the inverse of what I listed above; (IE: larger stomach and waste than hips, asymmetrical face, no breasts, etc...)
Best of luck to you, and remember, no matter how strikingly beautiful the woman or man, if they are shallow, and a bitch/asshole, they may find a many lovers to sleep with them, but real love and true acceptance with a partner will often elude them indefinitely...