"Lucid Dreaming" is the experience of knowing that you are dreaming while it is actively happening and gaining some control over what happens in the dream. What makes this exceptional is that most people are unable to consistently lucid dream, due to the realization that a dream is occurring causes the dreamer to instinctively wake up.

"Levelling" is a term I use for when , while -in- a dream, you go to sleep and dream another dream. It's like adding another level (or diving into another level) of the dream world.

The movie "Inception", aside from stealing an entire storyline from the Donald Duck comic book series, was a wonderful depiction of the use of both Lucid Dreaming AND Leveling.