I keep a grimoire of stuff concentrated in a Word doc. That's where most of my info goes. Then, I have a single text file where I write down various small ideas, which is generally snippets of dialogue, a chapter idea or a scene.
My Word doc is my baby though. I keep it neatly organized as I can (well with lots of TODO notes) so the navigation sidebar looks nice... (´_`)And so my writer can actually find shit! It pretty much has all info about the world in it.
Doesn't that look nice? I'm way too into aesthetics of a sidebar. But, that's how I organize my stuff.
My actual concept art though is all over the place. It sits in my general art folder with the 5000 pieces I've done. Not good with that. I've recently started to organize my character design folders so I'm getting there.