It refers to the influence of others on an individual’s behavior. Human behavior is influenced by other people in countless ways and on a variety of levels. The mere presence of others—as co-actors or spectators—can stimulate or improve one’s performance of a task, a process known as social facilitation (and also observed in nonhuman species). However, the increased level of arousal responsible for this phenomenon can backfire and create social interference, impairing performance on complex, unfamiliar, and difficult tasks.

Overt, deliberate persuasion by other people can cause us to change our opinions and/or behavior. However, a great deal of social influence operates more subtly in the form of norms—acquired social rules that people are generally unaware of until they are violated. For example, every culture has a norm for “personal space”—the physical distance maintained between adults. Violation of norms generally makes people uncomfortable, while adherence to them provides security and confidence in a variety of social situations. Norms may be classified as one of two types: descriptive and injunctive. Descriptive norms are simply based on what a majority of people do, while injunctive norms involve a value judgment about what is proper and improper behavior.



Both conformity and compliance are attempts to adhere to social norms—conformity occurs in response to unspoken group pressure, as opposed to compliance, which results from a direct request. Research has shown that conformity is influenced by the ambiguity of a situation (people are more apt to go along with the majority when they are uncertain about which course of action to pursue), the size of the majority, and the personal characteristics of the people involved, including their self-esteem and their status within the group. A person may conform by acting in accordance with group norms while privately disagreeing with them (public conformity) or by actually changing his or her opinions to coincide with those of the group (private acceptance).

In contrast to compliance, which characterizes behavior toward those who make direct requests but have no authority over us, obedience is elicited in response to a specific demand by an authority figure.

Another type of social influence that can lead normal people to engage in cruel or antisocial behavior is participation in a crowd or mob. Being part of a crowd can allow a person’s identity to become submerged in a group, a process known as deindividuation. Contributing factors include anonymity, which brings with it a reduction of accountability; a high level of arousal; and a shifting of attention from oneself to external events, resulting in reduced self-awareness.

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To summarize, social influence is the influence of others on an individual’s behavior. The mere presence of others—as co-actors or spectators—can stimulate or improve one’s performance of a task. Conformity, compliance and crowd or mob are three forms of social influence. Conformity occurs in response to unspoken group pressure, as opposed to compliance, which results from a direct request.
The techniques for gaining compliance rest to some degree on six basic principles – friendship/liking, commitment/consistency, scarcity, reciprocity, social validation and authority.

Sources:
Gale Encyclopedia of Psychology
Social Psychology, 12th Edition by Robert A. Baron, Nyla R. Branscombe, Donn R. Byrne, Gopa Bhardwaj (Click for eBook)
https://www.boundless.com/psychology/textbooks/boundless-psychology-textbook/social-psychology-20/social-influence-104/compliance-395-12930/