The id is the name that psychologist, Sigmund Freud, gave to the part of the mind that expresses the body's instinctive drives such as hunger, thirst, and sexual desire. Freud understood the mind to be organized in three parts: the id, the ego and the super-ego. His work formed the foundation of modern psychology so, even though subsequent psychologists frequently did not agree with Freud's theories, they respect him and use his theories as starting points from which their own develop.
The id, according to Sigmund Freud, is present at birth. It operates at an unconscious level to express and then satisfy the body's basic needs. One could say that the id is responsible for a person's most primitive, instinctive desires. The id does not moderate its desires based on any social or moral factors.
The ego, a second part in Freud's theory of the way the mind is organized, develops early in life. The ego can learn to measure the consequences of what the id desires. These are learned consequences that can be either social or pragmatic in nature. Once the consequences of satisfying the id have been considered, the ego guides the person to make decisions about whether and when to gratify the id. Until the ego develops, a person cannot easily handle delayed gratification.
The third part of the mind, according to Freud, and the last to develop, is the super-ego. Most people develop their super-ego by the time they are five years old, according to Freud. You could equate the super-ego to the conscience because the super-ego provides a person with the guidelines for making judgments about whether what the id wants is right. The super-ego is capable of a more nuanced evaluation than the ego. The super-ego imposes ideals and morality on what the ego and the id seek.
The description of the mind as made up of the id, ego, and super-ego provides the framework for Freud's larger psycho-analytic theory pf personality. He formulated this theory after doing extensive work with patients who were suffering from "hysteria." He discovered that trauma can result in psycho-somatic ailments for which no medical treatment will produce cures. However, counseling that led the patient to cope with the actual roots of the trauma, could result in healing. One such patient, Bertha Pappenheim, became the case study that Freud used to write "Studies in Hysteria" in 1865.
Sigmund Freud lived from 1856 to 1939. He was an Austrian physician who fled the Nazis with his family because he was Jewish. He died of cancer in England. His influence so changed the practice of psychology that he has come to be known as the "father" of modern psychology. He taught that some conditions do not have physical causes. He treated these conditions with a new therapy called psychoanalysis. He also suggested that people mature in a sequence of psycho-sexual steps. All of his ideas start with his understanding of the mind as id, ego, and super-ego.