Basically, a writer is someone who writes for a living, creating fiction (e.g. novels, short stories, scripts for films, theatre and television), non-fiction (e.g. biographies, autobiographies, web content and reference books) or poetry. They often work as freelancers.
Some writers are commissioned to write television scripts, whilst others will write articles for magazines and newspapers. Other writers may work as ‘ghostwriters’ (writing on behalf of someone else, under their name). Novelists and writers of non-fiction books tend to spend their time working on one single project, which takes a lot longer to produce.
A writer usually needs to do research, writing, editing, fact-checking, adapting and rewriting. More popular writers may even need to attend book signings or give press interviews for publicity purposes.