Freud divided the mind into three provinces, not necessarily anatomical, but theoretical:  the Id, the Ego, and the Superego.  (These unfortunately latinate terms are klunky translations of the much more simplistic and direct German ones, which translate more literally into the "It", the "I", and the "Above Me" or "Higher I". )
     The main difference exists between the id and the ego.
     Id:  locus of primary, primitive drives.  Your id is what steers you toward that car on the showroom you know you can't afford or your eyes toward that person across the room even if you're there with your significant other.  The id operates under the pleasure principle, meaning it has no regard for reality, constraints, or consequences.  A wild sexual fantasy or dream is pure id.
     Ego: balances the drives of the id against the reality of the world.  More organized than the id, the ego attempts to avoid displeasure and pain.  Patients with good control of their impulses and ability to tolerate difficult emotional challenges are said to have good "ego strength."  This is where Freud felt the real action was:  viewing it in somewhat neurological terms, he described it as critically involved in self-preservation of the organism through memory, awareness of stimuli, and making changes in the external environment to gain advantage.  The ego also can delay or discharge various impulses of the id, leading to release or tension.
     Superego:  locus of the internalized moral values, prohibitions, and ideals of the person.  The superego is what compels men to climb out of foxholes under fire to pull a wounded comrade back to safety.  It also is what probably drove you to attend medical school or become a physician.  It is the repository of your ego ideal, your idealized self, the self you want yourself ideally to be.  The superego is also where your conscience lives and is responsible for the experience of guilt.

[source:  Kaplan, p. 140-1]
 

     Central to Freud's theory was the idea of an Oedipus complex.  Based on the Greek tragedy Oedipus Rex, in which a boy banished as an infant by his father, the king, comes back and murders his father - unaware of his identity - at a crossroads, then marries his mother - also unaware until too late of her identity, the Oedipus complex posits that 3-5 year-old boys go through a crisis in which they seek to seduce their mother while fearing retaliatory castration by their father.  Successful resolution of this conflict involves acceptance that mother is unattainable and belongs to father, but that one day a mate might be found who will be a substitute for the relinquished mother.
     The Oedipus complex is perhaps one of the most controversial and widely criticized aspects of psychodynamic theory.   The female equivalent is more fuzzy and feminists and others have objected to the phallocentrism of his ideas (why, for example, should a girl suffer penis envy; why shouldn't a man suffer breast envy?)
     Further criticism of psychoanalysis developed during the mid-twentieth century, when it was observed that a number of cultures had no recognizable Oedipal complex. This called into question the Eurocentricity of Freudian ideas.   Freud's goal in formulating his theories of what drives people to do the things they do was to create an all-encompasising theory of human behavior and emotion, something akin to the universal laws of thermodynamics and physics that were being developed and popularized during his lifetime.  (Indeed, many of his earlier models viewed the psychic apparatus in hydraulic or mechanical terms; "psychical energy" was "absorbed" or "discharged".)
 

     Many Freudian concepts continue to be useful, however. For example, the idea of personality disorders representing a collection of maladaptive defenses (maladaptive in that they no longer serve the survival function they might have had in a traumatic childhood) is one shared by many researchers in the field, regardless of theoretical orientation.  Psychoanalytic concepts such as denial, rationalization, and acting out, although at times applied pejoratively and indiscriminately to any difficult patient, shed some light on what were otherwise incomprehensible, bizarre behaviors, and offered guidance in intervention.
     In addition, thinkers who advanced the work of Freud to the interpersonal realm, such as Harry Stack Sullivan and Karen Horney, or to the concept of object constancy, such as Melanie Klein and Heinz Kohut, have enriched our view of  human suffering.  Kohut also stressed the importance of attempting to understand and empathize with the patient's inner world.   Empathy, however, does not preclude the therapist from confronting the patient when the patient is engaging in destructive or counter-therapeutic behavior.
     Much of the research done on psychodynamic psychotherapy is naturalistic and suffers from a lack of randomization, control groups.
     Some argument might be made for brief psychodynamic psychotherapy, consisting of 10-12 sessions over a 6-12 month period, originally thought to be inappropriate for severely personality disordered patients, but now viewed as helpful, especially if therapy focuses on current problems, is solution-oriented, and limited in its scope (the patient is not encouraged to develop strong transference feelings).  Brief psychodynamic psychotherapy can also be given intermittently over a long-term basis (Silver, 1985).
     The objective of any psychodynamically oriented psychotherapy is to make the unconscious conscious, allowing previously repressed conflicts, wishes, fantasies, and drives, to be dealt with and in some way mastered.  The underlying assumption is that insight will lead to some meaningful change:  for example, a person who becomes aware of how much angry men remind him of his father and leave him paralyzed with fear may become more functional in the presence of angry men.