Writing is writing, typing away at the computer to develop a story and characters, draw out emotions, explore human nature. Seems a simple enough concept, right? But there's so much that goes into a book that isn't writing. Research isn't writing. Doing character sketches isn't writing. Storyboarding isn't writing, and neither is plotting, really, unless you plot as you write. Actually, this is pretty much what I do. It's called seat of the pants writing (I prefer soar with the wind), and even I don't particularly recommend it because it typically means more revising later. Speaking of which, going back and editing what you've already written isn't writing. At least, it doesn't increase your page count, not by much anyway. And doing promotions for your books isn't writing -- in fact, of everything that can interfere with your writing time, this is probably your biggest enemy, other than those friends who like to call "just to talk, because they know you're home and not doing anything."
So when is writing writing? When you keep your butt in that chair and your fingers moving over that keyboard. Even if you're groaning out loud over what your seeing on the monitor. One of my biggest downfalls is my instinct (i.e., head-bangingly obsessive need) to have it be perfect the first time around. I'm learning (i.e., forcing myself) to let it be not so perfect (i.e., crappy) in the first draft and accept that it IS just a first draft, and that it's much easier to edit later than have blank pages and a looming deadline.
Oh, my other pitfall? Spending too much time on the Internet and reading email. Later! 