Carl Gustav Jung was a practicing psychologist that derived his theory of psyche from the field of psychoanalysis and expanded on the idea of subconscious. Jung's contribitions include archetypes, colloctive unconscious, interpretation of dreams using mythology, a concept of emerging consciousness in primitive cultures and many other ideas. 
The archetypes of jungian psychology are considered to be his most important development. They expand on the general idea of various entities acting separately in a single person's psyche. Fundamental archetypes are the Persona and the Shadow, representing conscious self and subconscious respectively. While the Persona contains everything that a person knows and understands about themselves, the Shadow contains a variety of repressed or unknowable memories and thoughts. A deeper explanation of the Shadow would need to throw light on the major archetypes that are included in it. Such as the Anima/Animus, Wise Old Man, the Child, the Mother, the Maiden. 
A simple summary will not suffice to understand the whole work of Carl Gustav Jung. However, he wrote a multitude of books about his discoveries you can read.
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