Physical beauty, or rather human beauty varies differently by culture, but there are some extremely common trends that tend to follow in most definitions of beauty that can be traced back to our primitive roots.
The most important indicator of female beauty is the Waist-to-Hip ratio.
On Female Hip Size
Hip size indicates pelvic size and the amount of additional fat storage that can be used as a source of energy.
On Female Waist size
Waist size conveys information such as current reproductive status or health status.
Of course with a larger pelvic size means less physical trauma during childbirth, greatly decreasing the chance of injury or death to mother and child as a result of procreation. Evolutionarily speaking, doing the deed is meaningless if viable offspring is not likely. Remember that human beings are a special animal in that we have an unusually long post-partem dependence on parents to become independent physical, mental, and emotionally mature adults. This in many ways is a direct result of our large brains needing time to develop as well as other physical limitations imposed on how large an offspring can be produced as a two legged creature.
So the pelvic size and fat comes into play that you look very well fed and are more likely to carry to term, produce ample milk for the offspring, and have vitality to raise and feed children after nursing.
A persons waist tells a lot about their overall health. The most obvious thing that can be told is telling if the female is pregnant or not. Other health factors can be told by looking at the waist, general fitness, core strength, obvious deformities or flaws.
From the source cited above:
We postulate that in westernized societies with no risk of seasonal lack of food, the waist, conveying information about fecundity and health status, will be more important than hip size for assessing a female’s attractiveness.
So in most modern western societies we tend not to suffer from diseases of poverty but diseases of indulgence and abundance. It is therefore deemed more attractive that a woman not have excess body fat around the waist however it is still acceptable and beautiful for women to have bodyfat around their hips and buttocks as these are physiologically the healthiest places on the body to store excess body fat, thus one is still healthy for having fatty hips.
On Male Attractiveness:
Women, on average, tend to be more attracted to men who have a relatively narrow waist, a V-shaped torso, and broad shoulders, are taller than they are, and display a high-degree of facial symmetry, and relatively masculine facial dimorphism. With regard to male-male-attractiveness, one source reports that the most important factor that attracts gay men to other males is the man's physical attractiveness.
Again this is a throwback to our primitive roots where men and women didn't live and operate within a society that gives them equal standing to take care for themselves. When a women was with child, pregnant or otherwise she was severely hindered to hunt, gather, and build. To survive they needed to not only find and copulate with a strong provider, they needed the ability to keep them around (sex) so that they can be provided for while taking care of a child. The cost of reproduction is almost negligible for males unless women had the ability to keep men interested in them. Of course evolutionary speaking from the male perspective the primitive desire for fatherhood was another huge factor as raising viable offspring is the end goal.
Attractive men tend to be tall, have strong upper body strength and narrow waists. This conveys strength, ability to build strong structures and hunt. Ability to defend from danger. They tend to have good skin, and symetric features conveying they are healthy and free of genetic abnormalities.