First clarify writer: Someone who writes prose or poetry for money.
That being said the job description of a writer can be complicated and is wholly dependent on what type of writer the person is.
Take J. K. Rowling of Harry Potter fame. Her writing includes fantasy and mysteries plus it is geared mainly to young adults. Other authors have a similar lineage of genre and target audience. Some do Paranormal Erotica while others do Political Thrillers still others do Erotic Paranormal Mysteries. Yeah I know that is a mouthful but I'm sure there is a writer out there who does that kind of writing. Now add an target audience and you could get Paranormal writing for Children, Young Adults or Adults. The Children's category is further broken down into various age groups and reading levels. Its a lot to consider when you are starting out. Oh I almost forgot there is the length to consider as well. Writing comes in various sizes from the 50 word poem to the 1000 page novel and you need to choose the right length for your idea and your skill.
I have written erotica, poetry, opinion pieces and am working on a series of short stories and hopefully a novella in the Science Fiction/Fantasy genre. Each genre requires a different way of writing while maintaining a specific pattern of events. Lets see if I can explain that better. Every story has an underlying structure that is the agreed upon norm for fiction. On top of this basic structure you must add your characters and their interactions. OK I am talking about fiction here, non-fiction and poetry follow different rules.
So the first decision a writer who wants to get published must choose first prose or poetry? This is dependent on where their perceived strengths are. For myself it is poetry first because that is how I started writing. But my second choice is the most crucial because while writing poetry is good for the soul it is prose that sells.
Prose is broken into fiction and non-fiction. Now my choice for steady income would be non-fiction because if you write something people really want to know then it will sell for a long time. Fiction is more of a flash in the pan but has the potential to be the most lucrative if you find the right stream. For instance right now vampires are the most interesting subject but a year or so ago it was zombies. For all I know in a few months it will be all about fairies or werewolves. Fiction is very hit & miss that way. Romance sells of course but there are a dozen different flavours of romance from the pre-teen first love stuff to hard core erotica. Likewise Mysteries and Science Fiction have many different flavours
Non-Fiction is a matter of finding a subject people or talking about or interested in. If you have knowledge in this area you are ahead of the game if not then you have to do the necessary research to make it credible before writing it. OK that sounds much simpler than it is but since self help books fall in this category you will forgive my cynicism.
Since I have no particular subject that I am particularly knowledgeable in and I find research a bit boring I would do better to focus on fiction. Once that decision is made I now need to choose a target group. This and the choice of genre are the most critical choices a writer can make, length is whatever it turns into after the proof reading.
Ok that doesn't sound right perhaps the chain of choice should be target group, length, genre. The target group is important because you do not write erotica for a child or young adult and you don't write a fairy tale for an adult (well not usually). Now I need to think on these choices.
Most of my writing has been for adults but this series I'm working on I wonder if it might not be better for the young adult group. Since my main characters are between 21 & 25 perhaps... I have already decided that I want to do it in a series of short stories to begin with so that I can give my characters some depth and build their world. I have only a vague notion of what their world is like as of yet. The genres because they are my preferred reading matter and are the types of writing I am most familiar with are the Science Fiction/ Fantasy categories.
I need to go back and map their world, then decide on the main characters besides the hero that will find prominence in the stories and give them lives and motivations. I am finding that the most difficult because when I write poetry there is no character except my own spirit.
When I think of their world what comes to mind is like a refugee camp peopled by several races. In my preliminary writing I have identified four races. Furless ones, the woulven, the tree climbers & the purple people all of which are hominids,. Each one requires a different environment but all of the necessary environments are available on the world where they find themselves.
I have not decided rather they know of each other or are insular. Nor have I decided what each race adds/brings in the way of skills and abilities although obviously they need to be able to interact at some point of the story so no obvious xenophobes. The other thing I am sure of is that it is about growing up and taking your place in your society.