For those of you not involved in the realm of the writer, two quick definitions, typically used to describe the type of writer one is:
1. Plotter - someone who uses outlines, plot diagrams, and other tools to extensively map their story before writing.
2. Pantser - someone who just dives in and wings it. As in "by the seat of your"
I've always considered myself more of a pantser than a plotter, though I have elements of both. My process is to do extensive worldbuilding and character creation, figuring out details, societal elements, past history ... though that doesn't mean that all of these will remain fixed when I start writing. With all this framework built, however, I already know the sources of conflict and likely reactions, so when I get down to the business of writing, I can wind it up and watch it go.
When I started plotting for Unnatural Causes, my first novel length work that was also a mystery, I was inspired by a panel of mystery writers I had seen. All of them claimed that they didn't have a firm idea whodunnit until late in the writing. This sounded like an awesome idea to me. I would build characters with solid motives, come up with a passel of clues ...
... and find that it utterly didn't work for me. About a quarter of the way through, I got completely stuck. I couldn't continue the characters' investigation and trickle out further clues without knowing where they were leading. So I stopped, thought it through, and made some choices.
Now I'm doing my first readthrough of the manuscript, and I realize how much "pantsing" hurt the opening portions of the book ... and even later on. I'm going to have to do more extensive editing than I typically like to do, tearing some stuff out, including a red herring that doesn't even make sense any more ...
So I think I have to concede: I'm more of a plotter than a pantser, if only because I would much rather have the story going in the right direction to start with.