An Extrovert is a person who tends to focus on the external world and people. People often associate being socially outgoing with extroversion, but that is a little too simplistic. Extroverts may be more outgoing in that they gain energy from other people and the external environment, and usually prefer to be with others.
Carl Jung developed the idea of the extrovert and extroversion in his theory of personality. The opposite of Extrovert is Introvert.