Use the word id when you talk about the most basic, impulsive part of a person's mind. A baby's instincts for food and comfort, for example, are controlled by her id.
In psychology, particularly in psychoanalysis, the mind is divided into three basic parts. The most instinctive of these is the id, the drives for pleasure, food, sex, and aggression. The id is the most chaotic and least organized part of the mind, controlled in a healthy person by the ego and superego. The word id is a Latin translation of the "Es" in Sigmund Freud's "Das Ich und das Es." Both Es and id mean "it."