People tend to believe that certain physical features of human beings make them beautiful such as slim figures, fair skin, big eyes, high cheek bones, small noses, long legs etc. They believe that in order to be classified as beautiful these features and others are important to have. In most cases the reasoning behind this notion is absent-- but most people don't normally think as deeply about beauty.
The truth is that what humans find physically beautiful in other humans is nothing more than a product of sexual selection and no different from, for example, the beauty of what a female bird sees in a male bird which has also been selected for. The saying, 'Beauty is in the eye of the beholder', holds true in all circumstances of finding beauty in others because it is our own preferences for physical beauty that governs what we believe to be beautiful.
In most cases, these preferences for physical beauty are quite narrow and common within a particular society. Humans, as well as other organisms, have innate qualities that encourage us to form certain preferences about the other sex. Females tend to be attracted to strong, well-established and robust males while males seek youthful (fertile) and modest females. These innate preferences have evolved for thousands of years before civilization and are a main source for what humans perceive as beautiful today. Our preferences for beauty can also be stimulated by culture, religion, music, movies, media etc. Religion and culture encourage individuals to go against their innate desires and follow ancient traditional ideas that set limits on how to think and live. Furthermore, media encourages people to desire something they cannot be or have. These stimuli skew our perceptions of beauty. These aspects, both societal and innate, directly effect what people perceive to be beautiful and this perception is used, in particular, to find a mate.
Today there are so many sources that effect our perception of what is beautiful in others. These sources tend to fixate on physical beauty and less on the beauty of our biology to the point where it has become difficult for humans to see the beauty in anything let alone other humans. We tend to look at the most minute details in others to find beauty but are most of the times brutally unsatisfied. Society today seems to have lost the meaning of beauty in a sea of criticisms, superficiality and egocentrism.
It is unfortunate that to be beautiful has become a type of contest or competition where it is something that must be attained in our lives because without it we are nothing. Throughout evolution it is not our bodies that have changed much but it is our minds that have been plagued by silly memes (ideas) that have been passed on through generations. As people change their views of what they find beautiful in the opposite sex they change their sexual desires as well. And this is what drives evolution.
So now that we have changed our evolutionary path what will come of the meaning of beauty? We will most likely pass on the memes of beauty today to our children and they will too adapt their preferences based on the common conception of beauty. But the important thing to remember is that we do not need to get caught up in the competition to be the most beautiful of all because the criteria are not fairly formulated and do not fit societal norms. In contrast, they are based on some random person's belief of what ought to be.