In the study field of sociology, a "peer group" is a group of individuals who share similar interests, age, social status, and background. The overall ideas that the majority share in the group are likely to be considered as "the norm" and unconsciously influence the other members of the group.
Individuals usually associate to peer groups during their adolescence, as it is a period where the individual, who is yet to define its personality, is subject to a wide variety of influences that will eventually end up in him or her associating to certain groups.