In the trouble of trying to find a coherent and fair definition of consciousness, philosopher of mind David Chalmers writes that “[w]hat is central to consciousness, at least in the interesting sense, is experience. But this is not definition. At best, it is clarification” .
I would perhaps agree with the best clarification of consciousness as “the subjective quality of experience.” It is in this way that we see (in the fundamental sense) our own perceptions, thoughts and actions as a whirl of causation and information processing, and yet even then we are still neglecting the internal aspect of being a cognitive agent. According to Chalmers, “[t]his internal aspect is conscious experience” .
However, when asking what makes a being conscious, Thomas Nagel uses the famous phrase that a being is conscious if there is something it is like to be that being. In other words, a being with a mental state only has consciousness if it has a qualitative feel – “an associated quality of experience” . These qualitative feels are also known as phenomenal qualities, or just qualia for short. The explanation or expression of this qualia is one of the most difficult questions residing in philosophical studies of consciousness.
As Eugene Stark from Stanford University writes in his essay “Against Dennett’s Eliminativism: Preserving Qualia as a Coherent Concept,”
When a pin pricks one’s foot, a sequence of physical events are initiated that cause a pattern of neuronal firings that somehow represents the sensory information of pain. Where, however, does the qualitative feeling of pain fit into the neurological processes involved in the perception of a pin prick? In other words, how is a physical state of the brain responsible for the confused and subjective quale of pain? 
Qualia can thence be understood as the qualitative character of experience, or “what it is like to experience something” . This is the problem when experiencing certain flavors, or even seeing certain colors. For instance, your qualitative experience of some given thing is exclusive only to you.
So, say that came into contact with an alien from another distant galaxy who has no conception of the idea of pain. He has never experienced pain, nor has he ever heard of it. You can explain to the alien all day long the notion of pain, and even the effects that it may cause when one is in pain. However, although the alien can have an exhaustive knowledge of the notion of pain to even the anatomical level (he may even get a PhD in neurophysiology), but he can never experience the sensation of pain.
-  David J. Chalmers, The Conscious Mind (Oxford University Press: 1996) pp. 3-4
-  Ibid., p. 4
-  Ibid.
-  Eugene Stark, see paper here. (p. 1)
-  Ibid., p. 1