Dreams: What one remembers to have fantasized while sleeping after waking up.
Dreams are interesting experiences that are not at all well understood. They are interesting especially because while dreaming - at least: so far as one remembers - one usually, though not always, does not know one is dreaming, and what seems to happen to one seems to be as real as ordinary life, except that dreams may incorporate many events one would not do or dare or care for when awake.
In any case: Those who dream a lot, know from their dreams a sort of parallel universe, presumably completely fantastical (since one usually can verify that while one dreamt in fact one slept, and nothing happened to one as one dreamt that happened), yet apparently as real as the experiences that make up ordinary life.
This is an interesting fact, and one that is interesting for philosophy and epistemology, since it easily leads to questions like: "How do you know ordinary life is not a dream, albeit one you haven't woken up from, so far?" and "Supposing dreams to be fantasies, what do they mean or imply about one's person?".
Chuang Tzu has a nice parable about dreams, and Wu wrote an interesting book about Chuang Tzu ("The Butterfly as Companion"), but the question I just posed, or the one Chuang Tzu posed - He dreamt he was a butterfly, and woke up. Now, how does he know he is not a butterfly who dreams he is a man? - is rather easily disposed of by noting that ordinary life is not like a dream, and to say it is or may be like it is to confuse a few things quite categorically, and that we have evidence about men dreaming they are butterflies, but no evidence at all about butterflies dreaming they are men.
Also, what dreams teach about the dreamer's person is a moot question as long as no one really understands what dreams are. What is certain, is that for ages interpreters of dreams have made a lot of money or got a lot of kudos by what were basically fraudulent fantastic explanations. (Freud is one example in a long list of frauds of this kind - and at least his name fits nearly perfectly: Nomen est omen.)
However, while the topic is interesting, I am no expert at all on it, even though I happen to have a degree in psychology: Personally, I dream very rarely.
This does not mean that I do not have the kind of brainwaves that neurologists have connected with the occurence of dreaming in people (which in fact I don't know, but I suppose I have them like almost everyone who was investigated), but that I rarely wake up with a memory of dreams.
Also, if I dream and wake up knowing I did - once in a few years, at most - my dreams are excellent 3-D full color movies, but not spectacular or strange, and indeed not much different of how I would behave or feel in real life, and are normally about persons I have known.
In any case, I know from girl friends whom I have asked that they dream often; that they may dream in black and white; and that they may dream that they dream, or be faintly aware that they dream while they dream. None of this is true in my own experience, but then I also never had a nightmare.