What is the social identity theory and can you explain it in an example?

Social identity consists of an individual's sense of who he/she is depending on the group to which he/she belongs. Groups characterize for giving the individual a source of self-worth and, of course, self-esteem, as they help people achieve a sense of social identity. In other words, a person who has social identity feels that he/she is part of the social world.

A good example of this would be a football team with which individuals identify themselves and feel like they are a single unity.