Sigmund Freud (1856 – 1939) was trained as a medical doctor (neurologist), but his significance lies in pioneering the non-biological approach to psychiatric conditions. Freud is known as the founder of psychoanalysis as a method of treatment of mental disorders as well as psychoanalytic theory, which attempts to explain human mental functioning as a whole. Art and science in the 20th century have been tremendously influenced by Freud's ideas, and especially his notions of the conscious and unconscious mind and his theory of personality.
What is personality?
“Personality” is a term that is commonly used but not often defined. “Personality trait” is understood as a stable characteristics of human behavior and personality can be simply seen as set of traits that makes a person unique . In more precise terms, personality may be defined as a dynamic and organized set of characteristics possessed by a person that uniquely influences his or her cognitions, motivations, and behaviors in various situations .
Freud's theory of personality
Freud proposes a complete theory of personality that explains behavior and individual differences between people. Freud's theory emphasizes the role of the unconscious mind and early childhood experiences. The main aspects of Freud's personality theory concern the drives, the topography and structure of the mind, defense mechanisms and conflict resolution [4, 5, 6].
All thought, emotion and behavior is driven by some form of psychic energy. This psychic energy can take, according to Freud, two forms: the life, or sexual energy, called “Libido” and the death, aggressive energy, called “Thanatos”. This vision of the main drives is deeply rooted in biology and biological instincts: the desire to mate, reproduce and protect one's survival. All human activity can be, ultimately, seen as a more or less sophisticated expression of this fundamental, biological desire. Incidentally, according to Freud all human behavior is meaningful: even the smallest instances of action can be explained and interpreted as they are determined by the inner workings of the mind. Freud's theory doesn't leave any room for random chance, accidents or free will.
Topography of mind
Freud believed that the source of the actual drives, desires and motivations is in the unconscious He was the first major champion of the role of the unconscious mind, to the degree that his name (and psychoanalysis as a whole) became almost synonymous with the study of the unconscious
An image of an iceberg is often invoked to illustrate the role attributed by Freud to the unconscious. The conscious mind, or all that people are aware of, represents the above-water part of the iceberg. The vastly larger under-water section represents the non-conscious, which includes tow sections: the relatively small pre-conscious (the material that one is not aware of at any given time but that can potentially became conscious) and the biggest unconscious proper. The unconscious is not accessible to the conscious mind in the course of normal functioning, but manifests itself in dreams, mistakes and slips and can be brought to light in the course of psychoanalysis. The unconscious is the source of all or most of the psychic energy that drives human behavior.
Structure of the mind
Freud proposed a three-part model of the human mind. Each of the three parts is ruled by different motivations and principles.
The earliest and the most basic part of the mind is “id”. Id works according to the pleasure principle and is an expression of the organism's basic needs, which are, during what Freud calls “primary process”, translated into wishes. Thus, the id is a representative of the instinctual, biological aspect of the mind. The id resides in the unconscious and id's wishes are often not consciously acknowledged beyond the early childhood.
“Ego” is the part of the mind that resides in the consciousness, and operates according to the reality principle. Ego connects the organism to the reality via the sensory apparatus and it is used to mediate between id's wishes and the reality. Ego is a problem solving module of the mind and it represents reason. The process during which ego devises the ways to satisfy id's wishes while taking into account the conditions of reality is called the “secondary process”.
The third part of the mind is the last one to be formed, not appearing until mid-childhood (approximately 7 years old). It is called “superego” and it arises as a result of the ego keeping record of the rewards and punishments meted out by child's carers, and correspondingly, the best strategies to fulfill id's wishes. Superego operates on the morality principle and it is formed of internalized (taken as one's own) punishments and warnings (conscience) as wells as positive models of behavior (ego ideal).
In the ideal situation, the ego is the dominant and strongest element of personality and is able to satisfy the (fundamental and biological) wishes of the id while keeping the superego placated. This should lead to socially acceptable but ultimately effective fulfillment of the id's desires. If id is the dominant part, the resulting personality tends to impulsiveness, and inability to defer gratification. If superego is the dominant part, the personality becomes rigid, judgmental and inflexible.
Ego's role of maintaining the balance between the external reality, id's needs and superego's demands leads to inevitable conflicts. Freud believed that some typical conflicts between the demands of id and superego are characteristic of different stages of child's development. Personality develops through the resolution of the so-called psycho-sexual conflicts. Id, ego and superego develop typical patterns of interaction. In some people, the conflicts characteristic of particular stage of psycho-sexual development may not be well resolved. This may lead to personalities fixated (stuck) at a particular, early stage of development (and thus the talk of “oral” or “anally retentive” personalities).
Conflicts between the demands of the id, superego and reality are a common feature of human existence. Freud proposed that when the conflict becomes irresolvable by the ego, and the anxiety and tension overwhelms the ego, it reacts by processing the conflicting demands (unconsciously) and blocking or distorting the less acceptable of id's demands.
Psychoanalysts described several defense mechanisms, among which the best known are  denial (simple blocking of events from awareness), repression (unconscious forgetting of threatening events), displacement (redirection of a negative impulse or desire from a more threatening to a less threatening target), projection (tendency to see one's unacceptable desire in others but not in oneself), rationalization (cognitive distortion of the facts, unconscious making of excuses), sublimation (transformation of a socially unacceptable desire into an acceptable or even productive form).
Freud's significance goes beyond therapy and even psychology as a whole. His ideas about the human mind, culture and civilization are amongst the most influential theories of the 20th century, affecting philosophy, sociology, medicine, art, literature and politics.
The scientific status of Freud's theories is debatable, and there is little evidence to the factual accuracy of any of his concepts. Psychoanalysis has been called “the most successful pseudo-science in history” and there is precious little evidence as to efficacy of psychoanalysis as therapy . And yet, psychoanalysis and with it, Freud's theory of personality, remain endlessly fascinating.
Freud's influence on modern psychology, especially academic psychology, is definitely on the wane, but much of the psychological research and theorizing that followed Freud has been shaped and influenced by Freud . Outside psychology, Freud's role as the first champion of the role of the unconscious and sexual motivations in human behavior is huge and his philosophical and social influence remains without parallel in the history of psychology.
Sources and further reading:
 Engler, B. (1995). Personality theories: An introduction (3rd ed.). Boston, MA: Houghton Mifflin.
 Ryckman, R. (2004). Theories of Personality. Belmont, California: Thomson/Wadsworth.
 Webster, Richard. Why Freud Was Wrong: Sin, Science and Psychoanalysis. London: The Orwell Press, 2005.
 Psychoanalytic & neoanalytic perspectives on personality. Retrieved on 4 Feb 2011 from http://www.wilderdom.com/personality/personality.html#Psychoanalytic
 Boeree G. (2009) Personality theories: Sigmund Freud Retrieved on 4 Feb 2011 from http://webspace.ship.edu/cgboer/freud.html
 Heffner Ch. (2004) Personality synopsis: Psychoanalitic theory. Retrieved on 4 Feb 2011 from http://allpsych.com/personalitysynopsis/freud.html
Freud's own works (very readable and eminently worth perusing for anybody with even slight interest in psychology or personality):
Freud S. General Introduction to Psychoanalysis. Martino Fine Books. 2009 reprint of 1920 edition.
Freud S., Brill A.A. (translator). The Basic Writings of Sigmund Freud (Psychopathology of Everyday Life, the Interpretation of Dreams, and Three Contributions To the Theory of Sex). Modern Library edition (1995).
Freud S., Strachey J. (Editor), Gay P. (Introduction). The Ego and the Id (The Standard Edition of the Complete Psychological Works of Sigmund Freud). Norton edition (1990)
Freud S., Strachey J. (Editor), Gay P. (Introduction). Beyond the Pleasure Principle (The Standard Edition of the Complete Psychological Works of Sigmund Freud). Norton edition (1990)
Freud at Project Gutenberg (including English translations): http://www.gutenberg.org/browse/authors/f#a391