Freud believed that the majority of what we experience in our lives, the underlying emotions, beliefs, feelings, and impulses are not available to us at a conscious level. He believed that most of what drives us is buried in our unconscious. If you remember the Oedipus and Electra Complex, they were both pushed down into the unconscious, out of our awareness due to the extreme anxiety they caused. While buried there, however, they continue to impact us dramatically according to Freud.
The role of the unconscious is only one part of the model. Freud also believed that everything we are aware of is stored in our conscious. Our conscious makes up a very small part of who we are. In other words, at any given time, we are only aware of a very small part of what makes up our personality; most of what we are is buried and inaccessible.
The final part is the preconscious or subconscious. This is the part of us that we can access if prompted, but is not in our active conscious. Its right below the surface, but still buried somewhat unless we search for it. Information such as our telephone number, some childhood memories, or the name of your best childhood friend is stored in the preconscious.
Because the unconscious is so large, and because we are only aware of the very small conscious at any given time, this theory has been likened to an iceberg, where the vast majority is buried beneath the water's surface. The water, by the way, would represent everything that we are not aware of, have not experienced, and that has not been integrated into our personalities, referred to as the nonconscious.