Dreams vs. Nightmares

So you know how you have these things called dreams?http://en.wikipedia.org/wiki/Dream I don't know, maybe you don't dream, but I certaintly do. And some of them are so extremely wierd that I don't even know how to begin or explain them. Dreams have allways facinated me. What causes them? How can they be controlled? What do they mean?
Sometimes when I have a nightmare, my dream-self will try to run away but I could never actually run. I would be running in slow-motion or never getting anywhere. Nightmares can cause people to wake up in the middle of the night in a cold sweat. When I was little, I used to have a nightmare about a traintrack. I (as a little girl form) would be standing in an old, adbandoned train track. Of course the track being adbandoned, no train would come. I walk along the rails and suddenly I am on a train. At first, it's pretty thrilling. The train is going at a fast speed. But then I see a figure in the middle of the track. That figure is tied up and gagged. When I realise who it is, I sceam my head off for the train to stop. "MOM!!!!!"  When I wake up from my nightmare I saw my parents by my bed. They told me they heard me screaming and asked what happened. Nightmares can be so cruel.
I had a really random dream one time and it included Narnia, Harry Potter, and attractive goth guys. I was in an underground cave hidden with Dumbledore. I guess I was passed out because I woke up (within my dream) and wandered out of the underground cave and I met Voldemort. Then I was walking down the street when I came upon a class of highschoolers. There were nerds and a lot of attractive goth guys. You know, the ones with the hair? I came to an outdoor public pool where I had a conversation with a nerd about Narnia.