can be groups or "packs" of people, most commonly high-school students, who are defined by the music they listen to, the clothes they wear, the hobbies they undertake etc. Think typical stereotypes such as emos, punks, goths, sluts, preps, geeks and the like. Each clique will often have a definitive style of clothing, music, hobby etc. Over time, the terms may become overused and this results in people having misconceptions about each of the said groups, or automatically classifying someone as a memeber of the group just because of what they choose to listen to, wear, or do.

A group of people such as friends. Often used when discussing stereotypes.
Stereotypical meaning: Group of goths, preps, emos, punk, etc.

a group of people who hang around each other because they belong to the same sterotypical group (ex- goths, emos, preps, nerds, skaters etc).
they hate rival cliques, like preps hate emos and goths, nerds hate jocks and skaters and goths hate everyone.
pretty much the stupidest thing in middle school and high school history.

A term used to say you are a Twenty One Pilots fan

Crew, set, group of friends.

Groups of people (generally in middle and high school) that generally act and dress similar based on factors such as wealth, music, etc. Examples of this are: preps, goths, skaters, posers, and anyone who labels themselves. While in middle and high school, these groups have many people in them, but it starts to fade away in the late high school days, when people look back on themselves and say: "Wow, that was retarted."