Carl Gustav Jung
Jung's unique and broadly influential approach to psychology emphasized understanding the psyche through exploring the worlds of dreams, art, mythology, world religion and philosophy. Though not the first to analyze dreams, he has become perhaps the most well known pioneer in the field of dream analysis. Although he was a theoretical psychologist and practicing clinician for most of his life, much of his life's work was spent exploring other realms: Eastern vs. Western philosophy, alchemy, astrology, sociology, as well as literature and the arts. Jung also emphasized the importance of balance. He cautioned that modern humans rely too heavily on science and logic and would benefit from integrating spirituality and appreciation of the unconscious realm. Interestingly, Jungian ideas are not typically included in curriculum of most major universities' psychology departments, but are occasionally explored in humanities departments.
Jung and Freud
Jung was thirty when he sent Sigmund Freud in Vienna his work Studies in Word Association. Half a year later, the then 50 year old Freud, reciprocated by sending a collection of his latest published essays to Jung in Zurich, which marked the beginning of an intense correspondence and collaboration lasting more than six years and ending shortly before World War I in May 1914, when Jung resigned as the chairman of the International Psychoanalytical Association.
Today Jung and Freud rule two very different empires of the mind, so to speak, which the respective proponents of these empires like to stress, downplaying the influence these men had on each other in the formative years of their lives. But in 1906 psychoanalysis as an institution was non-existent. And Jung, who had become interested in psychiatry as a student by reading Psychopathia Sexualis by Richard Krafft-Ebing, professor in Vienna, now worked as a doctor under the psychiatrist Eugen Bleuler in the Burghölzli and became familiar with Freud's idea of the unconscious through Freud's The Interpretation of Dreams (1900) and a proponent of the new "psycho-analysis". Freud at that time needed nothing more than collaborators and followers to validate and spread his ideas. The Burghölzli was a renowned psychiatric clinic in Zurich and Jung an aspiring young doctor there on the rise. Another problem Freud had was that his slowly growing followership in Vienna was almost exclusively Jewish and Eugen Bleuler and Carl Jung were not.
In 1908 Jung became editor of the newly founded Yearbook for Psychoanalytical and Psychopathological Research, the following year Jung traveled with Freud and Sandor Ferenczi to the U.S.A. to spread the news of psychoanalysis and in 1910 Jung became chairman for life of the International Psychoanalytical Association. While Jung worked on his Wandlungen und Symbole der Libido (Symbols of Transformation), the tensions between him and Freud were rising, the nature of libido and religion playing an important role. In 1912 these tensions came to a peak, when Jung felt severely slighted by Freud visiting his colleague Ludwig Binswanger in Kreuzlingen without paying him a visit in nearby Zurich, an incident Jung at the time and in his autobiography referred to as the gesture of Kreuzlingen. Shortly thereafter Jung again traveled to the U.S.A. and gave the Fordham lectures, which were published as The Theory of Psychoanalysis and while they contain some remarks on the dissenting view of Jung about the nature of libido, if you read them today you'll be surprised to find largely a "psychoanalytical Jung" and not the Jung we've become used to in the following decades.
Jung and Freud personally met for the last time in September 1913 for the Fourth International Psychoanalytical Congress in Munich. Jung gave a talk on psychological types, the introverted and the extroverted type, in analytical psychology. This constituted the introduction of some of the key concepts which came to distinguish Jung's work from Freud's in the next half century.
The year before a strange incident had happened in the same city, when Jung and Freud met there with others in November 1912: At lunch there was a talk about a new psychoanalytic essay on Amenhotep IV, who introduced monotheism in ancient Egypt and who apparently had his father's name erased on all documents after his death. Relating this to actual conflicts in the psychoanalytic movement, Jung explicated his view on this, when Freud suddenly fainted and Jung carried him to a couch.
In the following years Jung experienced considerable isolation in his professional life, exacerbated through World War I. His Seven Sermons to the Dead (1917) reprinted in his autobiography Memories, Dreams, Reflections (see Jung bibliography) can also be read as expression of the psychological conflicts which beset Jung around the age of forty after the break with Freud.
Jung's primary disagreement with Freud stemmed from their differing concepts of the unconscious. Jung saw Freud's theory of the unconscious as incomplete and unnecessarily negative. According to Jung, Freud conceived the unconscious solely as a repository of repressed emotions and desires. Jung believed that the unconscious also had a creative capacity. The collective unconscious of archetypes and images which made up the human psyche was processed and renewed within the unconscious. In effect, Jung's unconscious, as opposed to Freud's, serves a very positive role: the engine of the collective unconscious essential to human society and culture.
Jung, Nazism and anti-Semitism
Though the field of psychoanalysis was dominated at the time by Jewish practitioners, and Jung had many friends and respected colleagues who were Jewish, a shadow hung over Jung's career due to allegations that he was a Nazi sympathizer. Jung was editor of the Zentralblatt fur Psychotherapie, a publication that eventually endorsed Mein Kampf as required reading for all psychoanalysts. Jung claimed that he did this to save psychoanalysis and preserve it during the war, believing that psychoanalysis would not otherwise survive because the Nazis considered it to be a "Jewish science" . He also claimed he did it with the help and support of his Jewish friends and colleagues. 
Jung also served as president of the Nazi-dominated International General Medical Society for Psychotherapy. Later in the war though, Jung resigned and joined the allied cause in the United States. In addition, in 1943 he aided the Office of Strategic Services by analyzing Nazi leaders for the United States.
However, whether Jung's later reversal, and his arguments to save psychoanalysis mean that he did not actually believe these things is still a topic of debate, with several controversial books written on both viewpoints.
Jung developed a distinctive approach to the study of the human mind. Through his early years working in a Swiss hospital with psychotic patients and collaborating with Sigmund Freud and the burgeoning psychoanalytic community, he gained a closer look at the mysterious depths of the human unconscious. Fascinated by what he saw (and spurred on with even more passion by the experiences and questions of his personal life) he devoted his life to the exploration of the unconscious. However, Jung did not feel that experimental natural science was the best means to understand the human soul. For him, a balance between hard science and the worlds of dream, myth, and spirit represented the most fascinating and promising road to deeper understanding.
The overarching goal of Jung's work was the reconciliation of the life of the individual with the world of the supra-personal archetypes. He came to see the individual's encounter with the unconscious as central to this process. The human experiences the unconscious through symbols encountered in all aspects of life: in dreams, art, religion, and the symbolic dramas we enact in our relationships and life pursuits. Essential to the encounter with the unconscious, and the reconciliation of the individual's consciousness with this broader world, is learning this symbolic language. Only through attention and openness to this world (which is quite foreign to the modern Western mind) is the individual able to harmonize his life with these suprapersonal archetypal forces.
"Neurosis" results from a disharmony between the individual's consciousness and the greater archetypal world. The aim of psychotherapy is to assist the individual in reestablishing a healthy relationship to the unconscious (neither being swamped by it--a state characteristic of psychosis--nor completely shut off from it--a state that results in malaise, empty consumerism, narcissism, and a life cut off from deeper meaning). The encounter between consciousness and the symbols arising from the unconscious enriches life and promotes psychological development. Jung considered this process of psychological growth and maturation (which he called the process of individuation) to be of critical importance to the human being, and ultimately to modern society.
In order to undergo the individuation process, the individual must be open to the parts of oneself beyond one's own ego. In order to do this, the modern individual must pay attention to dreams, explore the world of religion and spirituality, and question the assumptions of the operant societal worldview (rather than just blindly living life in accordance with dominant norms and assumptions).
The collective unconscious
Jung's concept of the collective unconscious has often been misunderstood. In order to understand this concept, it is essential to understand his idea of the archetype, something foreign to the highly rational, scientifically-oriented Western mind; the collective unconscious could be thought of as the DNA of the human psyche. Just as all humans share a common physical heritage and predisposition towards specific physical forms (like having two legs, a heart, etc.) so do all humans have a common psychological predisposition.
However, unlike the quantifiable information that composes DNA (in the form of coded sequences of nucleotides), the collective unconscious is composed of archetypes. In contrast to the objective material world, the subjective realm of archetypes can not be adequately understood through quantitative modes of research. Instead it can only begin to be revealed through an examination of the symbolic communications of the human psyche—in art, dreams, religion, myth, and the themes of human relational/behavioral patterns. Devoting his life to the task of exploring and understanding the collective unconscious, Jung discovered that certain symbolic themes exist across all cultures, all epochs, and in every individual.