lol wow what a broad question. dreams are something every creature does. a dream can be classified as a point where your imagination takes over and your mind slips away from waking life [hence day dreams] it's been about 5 years since i studied dreams, what causes them i'm not sure, and i don't think many psychoanalysts know either. every night people go through dream cycles of 90 minute cycles [if memory serves it's 90 minutes] of REM and non-REM. in non-REM you're in your deepest sleep and your most vivid dreams occur. everything slows in your body, your muscles freeze [sleep paralysis] and your body goes to work repairing yourself.
dreams can be little amusing things you're imagining, or your mind giving you subtle hints about events. since your brain is the only part that never gets a break, this is why many people feel you should meditate, clearing your mind slows it down, and helps it focus better, since it is said the mind is 'a drunken money who has been stung by a bee' it's hard, but can be achieved and this results in better health.
dreams also are important to health. a study was done to see the effects of dreams on the body. volunteers were aloud to sleep their normal 6-8 hours, but were woken up right before entering non-REM. Even though they received the right amount of sleep they normally had, the lack or non-REM left them irritable and weak.
dreams are made by your subconscious mind, you cannot make a dream happen, unless you've been awake far too long and you micro dream, your conscious mind slips away and the subconscious takes over. there is a difference between a dream and imagination. while i don;t wish to say you cannot control a dream, because you can, you can tell yourself 'i'm going to imagine i'm a samurai' but you can't tell yourself 'i'm going to dream i can fly'. there are some people who say you can, right before bed, tell yourself what you'll dream about, but it's the subconscious mind that makes the dream happen.
long and winded, if you want any more information, look up Carl Jung [not Freud, he's a creepy perv that's been disproven on many things.]