Writing for Communication
Writing can be a profession, a hobby, a past time or a necessary means of communication. Before internet and telephone and instant communication, writing letters was the only means people had to communicate with others over long distances.
Today, writing is still a necessary means of communication, but the internet changes the way writing is used to include email, instant messaging, blogging, and more. Cellular phones with text messages developed a new type of writing called 'text speak'. Even with electronics, some form of written communication still exists, so writing is an important skill to have.
Writing as a Career
Some people write for their career, and this can include anything from technical writing, computer code writing and nonfiction instructional writing all the way to science fiction, fiction and novel writing. The internet brought forth a new type of writing: web content writing. Web writers have to consider things like how to write effectively for the reader while also using skills like Search Engine Optimization (SEO) and other things that make web content easily findable by internet searchers.
Nonfiction and fiction writers can also write for print publications. These generally pay more than web content writing sites, but are much harder for a writer to break into. Print markets for article writing is one version of nonfiction writing, while there are also nonfiction books and instructional manuals.
Fiction writing is an entirely different type of writing career, that is more writing for entertainment, but also writing for money too.
Writing for Entertainment
Fiction writing is writing for entertainment purposes, even though some writers who write fiction actually make a very decent living. Writing fiction can be a painful and pleasurable experience. If you're interested in novel writing, you can read my at the link and you might be interested in trying your hand at NaNoWriMo.