Well I've pretty much got the plot of my story and the course of events right to the end sorted out. The question I have is whether it would add to the story and make it flow more if I wrote down backstories/biographies for my characters and the world it is set in before I wrote the story itself. I've been thinking over these backstories in my head in order to develop the plot, but is it better to write these down or just keep them in my head?
Personally, I wouldn't bother writing them down unless they are tied in with the story somehow. You created the story and characters, right? So you'll know them as well as you know yourself. Saying that, if you think it will help make the characters feel more real during your writing of the story go ahead. Though you should ask yourself whether you really want to spend so much time working on this when you could be writing or editing your story
Don't put them in directly, but backstories are a good thing. Have them in your mind, so that you can understand your characters and their actions/reactions more thoroughly. Also, it doesn't hurt to weave bits of backstory into the story itself, as it will show a deeper character, and give hints of a richer story.
I write everything down. I'm one of those mad people who has to have everything ready for writing. Yes, I can write bits and pieces from here and there, but to write the full story, I need character backgrounds and everything written so I can go back to them every time I need encouragement.
I personally write my ideas down, but if you read my full post, I've included a little advice for you as well. I searched how to start a story or novel and found a great website that gave me a lot of help in the creation of my current project. It explained that you should have some idea of a backstory for your characters, and it also said to "interview" them. "Ask" them their ambitions, disappointments, strengths and weaknesses, etc., and then to somehow incorporate this into the story. Just a little advice to you. I type most of my ideas in MS Word and save them, that way I can always refer back to them. But, you do whatever works for you. If you think you can remember your ideas, that's awesome! But you can always write them down and go back to them if you know you're going to possibly have a busy day or soemthing and forget them. It depends on what works best for you!
When you write your story, you are writing the backstory down to the degree necessary. By the time you are done revising, you will know the character thoroughly. If you write down backstory separately, you may end up constraining your character's development. You also risk contradicting what is in the story, especially after revision reshapes you characters. If you have ideas about places you want to take the character, by all means jot them down. But treat all such notes as subject to change. That's my opinion, of course. I feel it is vitally important to remain flexible with your characters' development.
Write down what you think you might forget, or what you might not be able to remember or keep straight in the story. (For example, say one character lost their parents when they were four. If this is something you won't be able to remember clearly, write it down in notes, because you wouldn't want to say that in one part of the story and then in another part say they lost their parents at age six!) Regarding the character histories themselves, write them down if it's something you feel like doing (they're your characters after all), but don't feel like you have to if you can remember it all in your head. And definitely DO NOT include detailed back histories and such in the story because this is what's known as an "info dump" and it throws off the reader. Just reveal what information you need to about each character as it naturally occurs in the context of the story. The writer might use a character bio or profile for themselves, but the reader is meant to find out about the character through the story itself. You didn't mention including the bios and histories in the story itself though, so I assume that's not what you were asking. Whether you write up their histories for your own use is up to you, but keep in mind that a good character's history is always changing. Hence you'd probably end up revising the histories a lot along the way.
Thanks for everyones advice, it's helped a lot. I think I'll write down information just so I don't forget and for my own general use. I'm the sort of person who has a lot of ideas and stuff in my head, but then forget them when it comes to writing. As for the story itself, the things I write down won't really have an effect on how I write it. I'll still try and give the reader information in small chunks to keep them reading.