It is important to understand that these terms are just words - words which we use to describe and discuss, so it is important not to place too much emphasis on the words themselves but to understand the theory.
The unconscious brain is generally the source of the ideas and sensations whereas the conscious brain is more accountable for reason and objective thinking.
Some people may frown against the idea that our brain has two separate centres, but remember what we just discussed about words. These are again just words used for the sake of communication – nothing more – nothing less. These two centres can for example be not separate locations, but types of nerve cells or different groups of nerve cells.
The fact is that a separation of two such areas in our brain is a reality. We know this from our experimentation with hypnosis which is able to suppress the conscious part of the brain. It can be discussed if hypnosis is able to suppress the conscious permanently or temporarily, but even if it is able to block it out for 1 second already proves the existence of two clearly separate parts of our brain – one, which could be suppressed and the other which acts in that time span.
In our everyday life we are not aware of the separation of these two parts of the brain as we experience all the sensations and events with both of them working in unison. However, if we want to understand our brain we must make the separation between the two.
The unconscious is said to be the primitive part of the brain and, supposedly, the conscious is the part of the brain which has evolved or developed from it, however, it is also known that the unconscious is very powerful and even claimed to be far more powerful than the conscious part of the brain, so one may wonder how a lesser brain can be the evolution of a much more powerful entity. These are questions which are yet to be answered.
What we know is that we are in control of our conscious part of the brain to one extent or another. Traumatic events and other stressful occurrences definitely make people take decisions which are far from objective or rational even though they are fully awake and healthy and not under influence of hypnosis or other device. So we only are in control of a portion of our conscious brain and it seems we are completely not in control of our unconscious part of the brain.
This is why this subject is so fascinating when you realize that we are almost fully not in control of ourselves and are driven by forces yet unknown to us.