An introvert generally prefers solitary activities to interacting with large groups of people. If you would rather work through your feelings in your diary than have a conversation, then you are an introvert.
Introvert comes from Latin intro-, "inward," and vertere, "turning." It describes a person who tends to turn inward mentally. Introverts sometimes avoid large groups of people, feeling more energized by time alone. The opposite of an introvert is an extrovert, who finds energy in interactions with others. Introvert can also be a verb, meaning literally to "fold inward," or to "turn inward," as when a child's personality seems to introvert once she starts elementary school.