A crowd is a large group of people gathered or considered together. People can be defined by a common purpose or a group of emotions, such as during political gatherings, sporting events or robberies (this is known as psychological crowds), or can be composed of many people to conduct business in busy areas. The term "crowd" sometimes refers to the lower class of the general population.

From the perspective of individuals and groups, social aspects are related to the formation, management and control of groups. In general, crowd control aims to persuade the crowd to agree with a specific point of view (for example, political assembly), or to include the crowd to prevent sabotage or mob behavior. Politically organised crowd control is usually carried out by law enforcement, but in some cases forces are used for particularly large or dangerous groups.

There are few studies on population type and membership, and there is no consensus on the classification of population type. Two recent scholars, momboise (1967) and berlonghi (1995), focused on the purpose of existence to distinguish between populations. Momboise has developed four types of systems: casual, regular, expressive and aggressive. Berlonghi classifies people as spectators, protesters or escapees to relate to the purpose of the gathering.

Another way to classify people is the emotional intensity system of Herbert Blumer, a sociologist. He divided the crowd into four types: Leisure Group, regular group, expressive group and performing group. His system is essentially dynamic, assuming that people change their emotional intensity over time.

The crowd can be active (mob) or passive (audience). Active people can be further divided into aggressive, escapist, habitual or expressive mobs. Aggressive thugs are usually violent and extroverted. For example, football riots and Los Angeles riots in 1992. The characteristic of the escapist mob is that many panic people try to get rid of the dangerous situation. When a large number of people fight for limited resources, there will be purchasing mobs. A expressive mob is anyone else who gathers for a positive purpose. Civil disobedience, rock concerts and religious revival all belong to this category.