The idea of a zombie, in a philosophical context, is the idea of a being that acts intelligently, and is physically the same as us, but has no consciousness. For example your best friend could be a philosophical zombie and you would never know it, because he, or she would act just as if they were conscious. Even an examination of their brain would not reveal any differences; all the physical aspects are the same as with a normal person. No one really thinks that there are philosophical zombies in the real world, but because the idea seems logically consistent some philosophers have concluded that the mind and body must be separate at some level. They argue that if the mind and the body were really unified then such an idea would be as inconceivable as the notion of a physical object that doesn’t have a spatial extent, and thus we should abandon our materialism or monism for some kind of dualism (property dualism is a popular choice for example).
If we truly wish to defend a materialist or monist position with respect to the mind-body problem we must show that philosophical zombies are in fact nonsense. There exist basically two ways to do this, the behavior response and the systems response.
The behavior response is to argue that anything that displays intelligent behavior is necessarily conscious. If we could defend this position then it would indeed show that the idea of a philosophical zombie in nonsense. Unfortunately it is hard to show that behavior, in principle, guarantees the existence of consciousness, for example see my earlier post concerning the Turing test. This might be a valid defense against the existence of real philosophical zombies, because we could argue that the only practically feasible way to generate intelligent behavior is as a result of consciousness, but in principle at least we could still be forced to admit the validity of dualism.
The systems response on the other hand is much more secure, and philosophers such as Dennett have in fact used it to refute the possibility of philosophical zombies. Once again we can divide the system responses into two types. One kind of a systems response is to argue that consciousness is a product of the activity of the brain. If this is true than philosophical zombies don’t make sense because they have the same physical brains as us, which should logically should generate the same kind of consciousness as us. Unfortunately it is hard to make this kind of response without arguing for some version of epiphenomenalism, the belief that the mind, although produced by the brain does not affect its operation, because if the mind is truly a byproduct of the brain it is hard to see how it could in turn affect our actions (assuming that the physical universe is casually closed). Thus we may reject this kind of response, and epiphenomenalism, because our ordinary experience indicates that we do indeed control our actions. A second kind of systems response then is to argue that the operation of the brain is itself consciousness. Many theories such as functionalism, aspect monism, identity theory, and descriptionism, would make this kind of argument against philosophical zombies. Once again if you accept this argument concerning consciousness then the concept of a philosophical zombie becomes nonsense, because we have stipulated that they are physically identical to conscious people, and thus their brains would indeed be operating in the same way, a way that guarantees, or is the same thing as, consciousness.
These resolutions to the problem of philosophical zombies may not seem entirely satisfactory, because the concept of a philosophical zombie was designed to discriminate between theories concerning consciousness, but the responses presented here use theories concerning consciousness to determine if the concept of a philosophical zombie is coherent or not. Such is philosophy.
If you want to read more on Dennett’s views concerning consciousness I would recommend his book Consciousness Explained (amazon).