One of the best movies I've seen in a while has been Inception, the new Christopher Nolan movie about a group of corporate thieves who enter into other people's dreams in order to steal company secrets. The plot is more or less incidental, but it does set the stage for a really fascinating exploration of dreaming, thought, and consciousness. What's great about the movie (aside from being lots of fun) is that it opens up a whole new world of possibilities within the human mind. Because we know so little about our brains, it makes you wonder 'what if?' and 'why not?'.
The whole premise of the movie is based on the knowledge of knowing when you are dreaming. Think about it--when we dream, what we see in the dream is what is accepted as reality. Even though a few things may be strange about a dream, you don't notice it at the time, and simply absorb it into the present dream consciousness. However, there does exist the much documented experience of dreaming while knowing you are dreaming, called Lucid Dreaming.
When you know you are dreaming, then you are able to control your dreams-- for example you can see someone who you haven't seen in years, do things that are impossible in real life, like fly or move through walls. Nightmares can be turned into healing experiences.... The possibilities are endless. The difficult part is that it takes quite a bit of training to have regular lucid dreams, although some people have them naturally on occasion.
Where to start?
Stephen LaBerge is one of the scientists who pioneered the study of Lucid Dreaming, and any one who is interested should check out his books. His website, the Lucidity Institute, gives lengthy excerpts from his books and other information. He gives practical advice on how to start lucid dreaming and gives tips on dream techniques. The first step is of course to remember your dreams, because if you don't remember you've had a lucid dream, then what use is it? The trickier next step is to realize that you are actually dreaming in the dream, and there are reality testing methods that can help you do this.
Of course, Lucid dreaming is very different from the dream states in Inception. There is no such thing as group dreaming, and research has shown that dream time more or less corresponds to the actual time passed in waking life. There are many other differences, besides.
It is a movie, after all.