There are many definitions of qualia, which have changed over time. One of the simpler, broader definitions is: "The 'what it is like' character of mental states.
Consciousness. Explaining the nature of consciousness is one of the most important and perplexing areas of philosophy, but the concept is notoriously ambiguous.
The Hard Problem of Consciousness. The hard problem of consciousness is the problem of explaining why any physical state is conscious rather than nonconscious.