In short, consciousness is the brain's user interface. Like a computer user interface, it hides a great deal of ad-hoc complexity inside a smooth conceptual surface that is designed to be as intuitive as possible. A software designer goes to considerable effort to come up with an appealing idea for how the user wants the computer to behave, and then does whatever is necessary to preserve this user illusion in the face of the peculiarities and limitations of the software and hardware platform that he is working with. See The User Illusion for a version of this viewpoint. See also the complementary interpreter theory.
It is difficult for someone who is not a programmer to fully appreciate this point. For a sample of how ugly it gets how quickly, consider the idea of a “file”. It seems simple: type some stuff into a word processor, say what you want to call the file and which folder to put it in, then save it. Later you can come back, navigate to the file, open it again, and your text is still there. Now, consider the FAT filesystem developed for the now obsolete and very unsophisticated MSDOS operating system. Just look at the description of how it works and the layers of complexity that have accreted. This complexity is only to remember the bits on disk. What goes on behind the scenes when you actually create those bits and then navigate to find them again using your mouse is astronomically more complex.
The FAT filesystem resembles the brain in another way: it also evolved. A considerable amount of the complexity and disorganization in the brain comes from the same process that gave us long filenames in MSDOS while preserving the ability of old programs to use short (but funny-looking) names.
Some may object to this metaphor on the grounds that it is just the latest in a long series of arguments that the brain is like whatever new piece of technology that comes along. When people first started to think of the mind as being a physical rather than magical or divine process, the said the brain was like a clock. When the telegraph came along, we were told that nerves were like telegraph wires. When mechanical calculators were invented, the brain was just like that. When servomechanisms where invented that had goal-seeking behavior, the new field of Cybernetics promised that understanding of the brain was imminent. When the first computers came along they were “giant brains.” Now we see that saying the brain is like a clock is simplified and distorted to the point of absurdity, and the telegraph and numerous other analogies have fallen from use. Aren't these computer analogies wrong too?
Well, no. As technology has advanced it has acquired more and more mind-like aspects, so these analogies have become more and more powerful. When we replace the clock and other mechanisms with the computer and software, we can see that a sufficiently complex system, even though it is deterministic or mechanical in every detail, can nonetheless exhibit complex and unpredictable behavior somewhat similar to a mind. Mind has emerged from the bits. More is different.