Although many people scoff at Sigmund Freud's theories, often without taking into account the sociocultural context surrounding his work, there are many aspects that have made their way into the common vernacular. As a result, they have also been distorted and diluted to the point that few people understand the original meaning behind these concepts. The ego is a prime example. This is originally one part of Freud's conception of the human consciousness (psyche to some).
Psyche: the mind, soul, or spirit of a person. It contains though, emotion, motivation and consciousness. It is separate from, and yet intertwined with, the body. From a philosophical standpoint, the psyche is the mystical energy the makes the body function. From a psychological standpoint, the word is used as a short-cut to illustrate all the properties and qualities of one's mind.
Id: the primitive part of one's psyche, according to Freud, that is present at birth. It represents the dual basic instincts of Eros (to create--not just lust or animalistic sexual urges) and Thanatos (to destroy--aggression and control). In infancy we have no concept of others or their needs; we are concerned only with our personal survival. All actions of infants are geared toward satisfying basic needs and pleasures. It is only as we age and engage in social interactions, as we grow in empathetic wisdom, that we move beyond this "id only" dimension. However, there are some individuals who seem to be more id-driven than anything else. Look up Freud's theories--his actual writings or a psychology textbook--for his ideas on how this extreme id shapes personality.
Superego: the socialized part of one's psyche, according to Freud, that develops through interactions with society, especially one's parents. It represents the internalization of the "rules" of society. When we follow the rules we feel pride in ourselves for doing what is "right." When we break the rules, we feel guilt and/or shame for falling short of perceived expectations. The superego is NOT an inflated sense of self, as was illustrated in the Duck Dodgers episode "A Lame Duck Mind." The superego is your conscience, constantly berating and correcting or patting on the back. Some individuals have an over-developed superego, often as a result of strict parenting or literal internalization of the "Dos and Don'ts" they encountered as children. Some people have a very underdeveloped superego, resulting in a very self individual who tends to not worry about the consequences of their actions, so long as their needs and desires are met. Then there is the extreme case of the antisocial personality disorder, which Freud believed was the result of no development of superego.
Ego: according to Freud, the part of one's psyche that develops out of necessity in order to maintain a balance between the id's desires and the superego's constant pressure for perfection. This term is intermingled with one's sense of self as well, so it can sometimes get to be a little confusing. In the Freudian sense, the ego is the moderator between the other two portions of the psyche. It is driving by the reality principle; it is the only portion of the psyche that is mostly conscious, so it makes the decisions. The ego constantly tries to find a way to satisfy the id while not violating the demands of the superego. Sigmund Freud believed that most psychological disorders, especially anxiety disorders, were the result of either the id or the superego pushing too far against the ego for the ego to handle it rationally. In a non-Freudian context, "ego" is used as a synonym for one's self-concept, which should not be confused with self-esteem or self-efficacy. I will create another post to differentiate between these "self" terms. Suffice it to say, your ego can be viewed as "who you think you are" and it can be fragile or strong, depending upon your personality and world-views. The ego, from Freud's view, however, is the part of us that is stuck dealing with reality. Sometimes you have to cut yourself some slack if you don't fully measure up to your superego or if you decide to indulge once in a while. The key to balance is moderation and understanding, not immobility or excuses.