Qualia (singular: quale) [pronounced: KWAH-lee-ah / KWAH-leh] is a philosophical concept which tries to describe the “what it is like” character of mental states.
 
Qualia are knowing what red is like, what pain feels like, what a rose smells like. A quale is a single unit of sensory perception.
 
These words are used to describe the connection (or disconnection) between the mind and body. For example, if one could create a perfect molecule by molecule copy of you (affectionately called a “zombie“), they may perceive the same things as you, but would they feel pain just as you do? Would they understand the smell of a rose as you do? Would their red-ness be the same as yours? Would they have the same qualia?
 
This is actually a very hot philosophical debate. If you interested in hearing more, here are the sources I checked out:

• Qualia from the University of Waterloo: A quick little definition.
• Quale - Wikipedia: A little more detail.
• Stanford Encyclopedia of Philosophy - Qualia: Almost too much information (not that there is such a thing).

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