Quick Definition: The behavioral expectations and cues within a social setting or social group.
In sociology, the social norms are “the rules that a group uses for appropriate and inappropriate values, beliefs, attitudes and behaviors.” These rules can be communicated verbally (explicit) or through subtle cues and subcommunication (implicit).
Depending on the type of social circle, failure to follow the rules can result in negative reinforcement from the social order. This can result in slight annoyance from other group members to severe punishments, as well as exclusion from the group.
For example, the corporate culture or corporate “social norm” of IBM in the 80s was the blue suit with the tie. If an engineer wanted to dress in flip flops and jeans, he may not be taken seriously in meetings or, worse yet, be asked to re-dress himself or leave the company. If an older man goes to an 18+ goth club dressed in a cardigan sweater, he may be implicitly breaking the social norms.
Interestingly, popular kids in high school and tribal leaders and those in power have higher bandwidth to set, establish, and bend social norms to their will. This usually comes from a place of belief combined with power to execute. An example can be seen here in the movie Mean Girls, written by Tina Fey. The film takes a comical view of the social norms that influence most American high schools: