Even though the model is "structural" and makes reference to an "apparatus", the id, ego, and super-ego are functions of the mind rather than parts of the brain and do not necessarily correspond one-to-one with actual somatic (physical, corporeal) structures of the kind dealt with by neuroscience.
The concepts themselves arose at a late stage in the development of Freud's thought: the structural model was first discussed in his 1920 essay "Beyond the Pleasure Principle" and was formalised and elaborated upon three years later in his "The Ego and the Id". Freud's proposal was influenced by the ambiguity of the term "unconscious" and its many conflicting uses.
The id comprises the unorganised part of the personality structure that contains the basic drives. The id acts according to the "pleasure principle", seeking to avoid pain or unpleasure aroused by increases in instinctual tension.
The pleasure principle is a psychoanalytic concept, originated by Sigmund Freud. The pleasure principle states that people seekpleasure and avoid pain, i.e., people seek to satisfy biological and psychological needs. The counterpart is the reality principle, which defers gratification (satisfaction, fulfillment, indulgence) when necessary. An individual's id follows the pleasure principle and rules in early life, but, as one matures, one learns the need to endure pain and defer gratification, because of the exigencies and obstacles of reality. In Freud's words, “an ego thus educated has...