Writing, of course, is obvious. One needs to know grammatical rules, syntax, diction, and all of those eloquent words higher ups use to describe language. That isn't to say one needs to know what it is that makes writing good. One can be a good writer without knowing the dictionary definitions of the devices he or she uses. In either case, though, one needs to know basic writing rules such as one speaker of dialogue per paragraph, appropriate paragraph breaks, attainable descriptions, that sort of thing. This is the thing that I expect most people associate with what it means to be a writer, but this is only a piece of the puzzle.
Outlining is another huge part. There are several ways to outline a story, of course, and they all fall under style of how the author wants to go about writing it, but without an outline the story will only have a basic form of where it should go. I've written a novel with no outline before, thinking that since I had the whole basic plot in my head I wouldn't have to worry. But outlining is crucial to the story because without a solid map of where to go the characters can literally get lost trying to achieve their goals. An outline will make certain that doesn't happen. (As a side note, this is my worst area, because I hate outlining. I'll make a rough outline of every chapter, and then use the chapter outline as a vague guide to follow as I manipulate the characters how I will anyway. Realistically I should update the outline as I'm writing to get a clear path to follow, but I just don't like it so I don't.)
The third part is something only a few people even have to worry about. Most fiction writers write stories of fictional people doing fictional things in 'real' places. That isn't to say that the setting in every story is a real place, of course, but when one is writing about a modern day doctor's office one doesn't have to think about what it would look like because it's a real thing. Every reader will already be able to easily picture a doctor's office. But some people don't like writing about the real world. If you're like me, by far the most interesting part of writing is the worldbuilding. This is the creation of nations, of wars, of creatures. Worldbuilding is making a world that is similar to our own, but is unmistakably different. To me, this is the part of writing that keeps me writing. I would never write novels about murder mysteries or anything that happened in the real world. At least, I wouldn't really enjoy it. Worldbuilding is getting somebody to imagine a piece of your mind. To me, it's what writing is all about because people can easily imagine somebody getting killed. But people cannot easily imagine a war of magic and dragons and other epic stuff, and writing is about implanting those seeds of creations into the minds of others. Some day, if people can recognize some of my characters and creatures without even really knowing who I am, I will have achieved my goal.
So, I find it strange that people write their stories in these worlds without really building them first. This is, in a very general sense, a part of outlining, but this isn't organization, it's creation. One needs to establish what people are like in this world, and what sort of things they want out of that life. If one writes about some peasant without knowing who the king is and what sort of policies he rules under, it may come back to haunt the author when that character meets the king and things don't really make sense. Yes, it is possible to make a benevolent king while having the peasant live under tyranny and hating the kingdom, but if that is established it must be explained. Is the king a figurehead? Who is really pulling the strings? How did this puppet master get there? What are his true plans? As a worldbuilder, one must go through these questions and answer all of them realistically in order to make a compelling world with which to tell a tale.
With all that being said, you may or may not know which things are your strong suits. Every writer must be competent at all of these, but some will, of course, need more work than others. If you've always wanted to write a book, but were never really sure how, maybe this could help. Do you wonder why your stories are so unbelievable or stale? Perhaps one's writing needs work. I'd say writing is easier than people think it is, but when one is missing one piece of this puzzle it seems impossible. It's not. Find that piece and you'll be good to go.