i assume you're talking about physical attractiveness. honestly, it's so abstract that you can't really put it into words. you could describe two girls to me exactly the same, but one might be pretty and the other ugly. science has some theories that attractiveness is largely based on facial symmetry. also, one's weight affects the shape and appearance of their face, being too skinny or too heavy can make you not look your best. and then some non-genetic factors come into play. personal hygiene, having a flattering/unflattering hairstyle, etc.
Guys might not realize it but personality is always first! But then its just little things like, nice teeth, and pretty eyes, and then of course its the "sexy things" like a nice ass and things along that line.