Carl Jung is a doubly paradoxical thinker. His thought is both about paradoxes and in itself paradoxical. Jung, unlike other psychoanalytic thinkers such as Freud and Lacan who focused on interpersonal subject-object relations, focused on the self contained individual psyche and its relations with itself. On the other hand, the emphasis he placed on the Collective Unconscious makes the subject for Jung anything but self contained. Jung held that the collective unconscious is no less, even more, important than Freud's personal biographical unconscious.
Laying on his sofa, Freud might ask you about your relationships with your mother. Jung might not be as interested in your mother as he is with the mother. Jung is not so much concerned with how concrete individuals populate our mind; he is more interested with how our mind is populated with abstract figures that represent primeval elements of human existence. What we are dealing with here are the gods.
Much like Freud, Jung divides the psych into three parts: the I (ego), the personal unconscious and the collective unconscious. Centering his thought on the collectiveunconscious is what distinguished Jung from other psychoanalytical thinkers and their view of the subject. For Jung, the collective unconscious plays a role which is no less and perhaps even more important than the personal unconscious in determining our personality.
According to Jung the collective unconscious is something inherited, meaning we are already born with certain ancient knowledge or mental content. The collective unconscious for Jung explains the continuity of culture and our sense of common experience with previous generations.
Jung's collective unconscious is populated by archetypes. Archetypes for Jung are amorphous shapes which are manifested with culture specific content. To understand Jung's concept of archetypes consider the fact that every human being has a mother, and every culture has to relate to the mother in some fashion or the other and assign her with meaning. For Jung, we are born with the idea of a mother (otherwise we wouldn't have been able to survive) that later takes shape in the form that native society perceives and represents the mother archetype. Jungian archetypes are such universal forms like the father, mother, hero, shadow and more.
The "shadow" archetype is one of the central concepts in Jung's theory. The shadow archetype for Jung somewhat resembles Freud's unconscious. The shadow is a form which takes on content perceived by us a negative. The more we are disengaged with the shadow the more it accumulates potency and causes damage to the psyche.