Groups of people may be identified by any common characteristic shared by members of the group. Any time two or more people have something in common, they can be said to have a relationship based on that common feature; they can then be identified by that commonality. Society, and its study through sociology, is dependent upon the application of information gained by studying these shared traits.
Physical characteristics are easy because they are visible. People may be grouped based on color of hair or eyes, on height, on bone structure. People are grouped based on skin coloring, shape of eyes, nose, or mouth, usually referred to as racial characteristics.
People may be grouped according to shared interests or activities. Sports teams, choirs, bands or orchestras identify groups of people as being members of the given group. Persons may be identified as being students at a particular school, employees of a specific company, residents of a certain country. Members of a specific club, religious group, or other organization are identified by that membership.
Common conditions can provide another way of identifying people. Persons may be identified as being cancer survivors, alcoholics, having a given allergy, and so on.