What is Lucid Dreaming?
The idea is simple: Lucid Dreaming is knowing you are in a dream while you are in a dream.
Although lucid dreams are usually very realistic, vivid and bright, and endow the dreamer with clear thought, NONE of these are required nor necessary in an actual lucid dream. For instance, I've noticed that when a bright light shines on the eyelids while lucid, the visual imagery in the dream can be reduced to a blur, virtual blindness. So, vividness is not guaranteed while lucid. Realistic is usually synonymous with vivid when relating to dreaming, but taken literally, lucid dreams can incorporate some very unrealistic imagery (I once fought a cartoon version of superman in a lucid dream). Further, you will not be lucid of everything in a dream, just like you are not aware of everything going on while you are awake. It is common to be lucid and still believe that the characters in your dream are the actual, real people they represent from the physical world. Also, it is very common that lucid dreamers are afraid of falling while flying high (as would be expecting in a virtual space with programs embedded specifically to activate a fear of falling).
A lucid dream is not entering into a parallel universe (or, if it is, supply evidence and win the Nobel Prize!).
Lucid dreaming happens to many people once or a few times spontaneously over the life-span. Usually, from the excitement of realizing they are in a dream, a person will immediately wake up. Because of this, few people spend much time in lucid dreams. Hopefully, as more and more people become interested in inducing lucid dreams, this will change and as more and more people explore this terra incognita we will learn a plethora (have to throw that word in every few years) of information about why we dream, and how we could dream.