Writers have over 28 genres and sub-genres from which to choose. The main categories are, of course, FICTION, NON-FICTION, and POETRY. Within Fiction, the sub-genres are mind-boggling and, in the past decade, cross-genre has become all the rage.
I mentor a young writers group, ages 12 to 20, and I recently gave them the list of genres in the article at http://cubits.org/writing101/articles/view/932/. The assignment was to look through the many choices open to a writer and choose one that interested them. I encouraged them to think outside their comfort zone.
Writing more than one genre gives an author some flexibility, and keeps one's writing from becoming stale and predictable.
I write fiction (romantic suspense, thriller, contemporary, young adult) and non-fiction (historical, travel, gardening, cooking, documentary) and I can honestly say, I am NEVER bored! And I'd like to think my readers feel the same way.