Qualia are internal, subjective qualitative states such as the redness of red, aesthetic experiences of beauty and revulsion, pain, happiness, boredom, depression, elation, motivation, intention, the experience of understanding something for the first time, etc. Such states are subjective and private and are distinct (though causally related to) physical and neural activities.
The experimentally accessible processes, such as projection of images on the retina and the resultant neural firings etc, are describable in terms of manipulation of symbols (typically binary states such as fired/not fired, matrices of pixels or strings of pulses). However, how these symbols and the processes that manipulate them give rise to qualitative experience is one of the major areas of difference between the materialist and Buddhist viewpoints.
To the materialist, all perceptions - sight, hearing, touch taste and smell - arrive in the brain as bitstreams, a sequence of 1's and 0's like the bitstream which is bringing this information to you down the telephone wire. The 1's and 0's are physically implemented as electro-chemical impulses of neurons. The neural nets within the brain process these raw bitstreams, firstly into data, then into information and finally into knowledge.
Buddhist philosophy has no difficulties with this process up to and including the point of generating information. However it points out that no mechanistic explanation appears to be able to bridge the gulf between information and knowledge, ie from symbols (whether on the printed page, or in the brain) to actual experience. There seems to be no bridge between the data about a rose, no matter how they are processed and arranged, and the actual subjective experience of the rose. The immediate knowledge of the rose consists (among other things) of the qualia of red, green, and the smell of its perfume, not to mention the very immediate and unpleasant sensation I get when I attempt to pick it up by its thorny stem.
The Buddhist does not doubt that the brain does some very sophisticated ordering of its incoming nerve impulses into the datastructures which are the objects of knowledge. But when all is said and done, those data structures remain as objects. They are not themselves knowledge, neither are they that which performs the function of knowing.
A datastructure by its very nature must have form. But according to Buddhist beliefs, the mind is formless and is capable of grasping any object of knowledge, including facts about the mind itself, which then become objects of knowledge in their own right. Consequently the mind is potentially unbounded.
Buddhist philosophy states that the the gap between information and knowledge cannot be bridged from the data-object side, it can only be bridged by the mind reaching out or going to its object (as it appears to do in certain quantum phenomena such as the 'spooky action at a distance' discussed in the section on quantum phenomena). Thus the mind is not a extension of the dataprocessing capabilities of the brain either in terms of hardware, datastructures or algorithms. It is something totally different in its fundamental nature from all of these.
There is no one-way chain of causation between neurological events and qualitative mental states. Rather the mind appears to impute or project reality over the contents of datastructures within the brain, much as a PC operator would impute text, graphics, windows etc over the two dimensional array of dots that comprise a computer display.
"Qualia ( // or //), singular "quale" (Latin pronunciation: [ˈkwaːle]), from a Latin word meaning for "what sort" or "what kind," is a term used in philosophy to describe subjective conscious experiences. Examples of qualia are the pain of a headache, the taste of wine, the experience of taking a recreational drug, or the perceived redness of an evening sky. Daniel Dennett writes that qualia is "an unfamiliar term for something that could not be more familiar to each of us: the ways things seem to us." Erwin Schrödinger, the famous physicist, had this counter-materialist take: "The sensation of colour cannot be accounted for by the physicist's objective picture of light-waves. Could the physiologist account for it, if he had fuller knowledge than he has of the processes in the retina and the nervous processes set up by them in the optical nerve bundles and in the brain? I do not think so." 
The importance of qualia in philosophy of mind comes largely from the fact that they are seen as posing a fundamental problem for materialist explanations of the mind-body problem... More
- Sean Robsville
Numinous Symbolism - Pagan, Buddhist and Christian
C J Jung, Buddhism, Tantra and Alchemy