Stephen King writes horror novels.
John Grisham writes legal thrillers.
Tom Clancy writes military/political thrillers.
Isaac Asimov wrote science fiction.
J. K. Rowling writes YA fantasy novels.
Robert Jordan and J. R. R. Tolkien wrote in the fantasy genre.
Terry Brooks writes fantasy novels.
Danielle Steel writes romance novels.
Agatha Christie wrote mystery novels.
John Sandford writes thrillers.
Funny how when one works and perfects their craft, they become known for it. Now, some of these writers (and countless others I haven't named) do write in other genres. I love Stephen King's stories, yet some of his best ones in my opinion are not horror (The Green Mile, On Writing). The same with John Grisham (A Painted House, Bleechers, Playing For Pizza).
Jim Collins in his book Good To Great talks about simplicity: getting good, being the best, at one thing. Do something well enough, with excellence, and you will define that genre or that segment of business.