Sigmund Freud is perhaps best known for his research and analysis into what he coined the Ego, Superego, and Id(or instincts). In modern times it's more commonly referred to today as Consciousness, Sub-conscious, and Unconscious respectively. The misconception that a human only uses 10% of their brain has become commonplace in today's world. However, numerous studies have been done to infer that we use far more, if not all of our brain at all times.
So where did this 10% myth come from then? Why do people believe we only use 10% of our noggin? The fact of the matter is that people are probably only aware of 10% of the going's on in there brain. Consciousness, although not immediately definable (in fact you'll find it quite hard to define exactly what consciousness is), can be described as awareness, wakefulness, or the control you have over your mind and your actions. However, these descriptions are still not entirely what Consciousness is, but they do help to form an idea of its nature.
The best way to describe how you use your brain is to imagine an iceberg floating on the water. What you see above the water is what you are aware of when you go through your day to day experiences. This represents your conscious life, and your awareness of self. Anytime you take time to think about a decision, or smell some roses, or do some kind of problem solving, you are only using the tip of the iceberg of your mind. This is the typical 10% usage of your brain, and the part we're the most comfortable using because it is what makes us unique.
The part just below the water, that you can barely see is known as the sub-conscious. These are the parts of you that you rarely notice, your habits and convictions. There's plenty of things that people do daily that are representations of their subconscious at work. When you bite your fingernails, or scratch your head, you probably don't realize you're doing it at first but notice the sensation of relief it might give you. This is a good example of the subconscious at work.
The last bit of the iceberg analogy is the part that you can't see, the part that traverses far too deep beneath the depths of the mind for us to access consciously. This part is known as the unconscious, and it is made up of every other part of your cognitive processes that make up you. The instructions to make your heart beat, your lungs fill with air, and digestion work are written here. It is also thought that the unconscious is where you go when you dream, though there's no way to objectively prove this(yet).
Not much is known about the unconscious since we can't directly view it. However, modern cognitive psychologists and neuroscientists agree that there is a lot more going on than what we think inside our minds. Recently in Scientific American Magazine there was an article about the brain's "Dark Energy", which is just another way to describe a phenomenon of increased brain activity when a person is doing seemingly nothing at all (daydreaming for instance). So to say we only use 10% of our brains is ineffective and wrong. The truth is we're probably only really aware of about 10-15% of what goes on inside our brain - which is probably something we should be thankful for.
Another thing to keep in mind about the mind (am I repeating myself?) is that we may not be 100% aware of our surroundings consciously, but our mind has no trouble processing and accessing all that is around us. Sensory information has to be processed somewhere, and that place is the brain. So, when you hear a sound, or see something out of the peripheral of your eye, that information is then processed in the brain, recorded to memory, and subsequently "saved" for further use. With the constant bombardment of sensory information that needs uploaded into our brain, you'd be hard pressed to say we use 10% of our brain. Interestingly, some Autism findings show that they have trouble processing certain sensory information, that their brain literally becomes overload, which is why they typically have fits and love repetition. Thought this is still just a theory.
Your mind is the largest and most powerful computer on the planet. To say you only use 10% of it at any given time is not doing it justice. It also makes me wonder why we have such a waste of space if that were true. However, most research into the subject suggest we do in fact use all our brain, that even when we're idle or daydreaming, our brain is making complex computations or planning strategies for possible future problems and situations. So give your brain a pat on the head, maybe a meditative exercise,and thank it for being such a well designed and dependable device - despite only being aware of a tenth of its processing.