Social influence is when the actions or thoughts of individual(s) are changed by other individual(s). Peer pressure is an example of social influence.

Social influence is not rare, occurring constantly as a process of socialization.

In the case of peer pressure, a person might be forced into doing something (such as going to an opera) he might not like but is "necessary" to upkeep the positive relationship with the other party, such as the family of his/her girl/boyfriend. The person could agree to the offering even if he hated it because of many reasons; maybe he is the possible inheritor of the family/person that asks him to come to the opera, the family could want to evaluate the person before letting their son/daughter get married with him/her, etc.

Social influence can also be described by the word Power (sociology), which means to possess the ability to embrace a person/group of people to one's own will. Usually people of good genes (looks), significant sums of money, good jobs and so on will possess social influence on other, "ordinary" people. So even if the person doesn't possess any "real" or political power but possessed the things listed above (good looks, money, etc.), he could persuade other people into doing/saying something.

An example would be movie stars, who don't (usually) possess any political power but are familiar to many of the world's citizens and therefore possess social status. They get a lot of media coverage, they are admired in television shows and they have many enthusiastic fans.