Sigmund Freud, founder of Psychoanalysis, had theory commonly referred to as the iceberg theory in which he proposed to split the human into three levels of consciousness.
He said that every human had an unconscious, a preconscious, and a conscious level to their minds. According to his theory the unconscious mind was the largest part, but also was the hidden or repressed part. This is shown in the picture above as it is below the water. Still even though this part was repressed according to Freud this part had the most influence.
The middle section was called the preconscious or subconscious and this was the part that was sometime acknowledged by the brain and other times was not. In the picture above this is show by it being right a the water level and bobbing up and down above and below the water line.
The final part was the conscious part. This is the part of our mind we are aware of on a daily basis and makes most of our decisions and thoughts. Still Freud said although this is the only completely visible part it is still mostly affected and influence by the unconscious lurking below.
Freud also described what parts of our minds and thoughts were found in each level of consciousness.
For more information on the levels of consciousness and what fall into each part check out these sites:
Topography of the Mind
Psych 101 – Topographical Models of PersonalityPsychoanalytical Psychology