A screenplay writer (also called screenwriter for short), scriptwriter or scenarist is a writer who practices the craft of screenwriting, writing screenplays on which mass media, such as films, television programs, comics or video games, are based. 

Screenwriting is a freelance profession. No education is required to become a professional screenwriter, just good storytelling abilities and imagination. Screenwriters are not hired employees but contracted freelancers. Most, if not all, screenwriters start their careers writing on speculation (spec) and so write without being hired or paid for it. If such a script is sold, it is called a spec script. What separates a professional screenwriter from an amateur screenwriter is that professional screenwriters are usually represented by a talent agency. Also, professional screenwriters do not often work for free, but amateur screenwriters will often work for free and are considered "writers in training." Spec scripts are usually penned by unknown professional screenwriters and amateur screenwriters. 

There are a legion of would-be screenwriters who attempt to enter the film industry, but it often takes years of trial-and-error, failure, and gritty persistence to achieve success. In "Writing Screenplays that Sell", Michael Hague writes, "Screenplays have become, for the last half of [the twentieth] century, what the Great American Novel was for the first half. Closet writers who used to dream of the glory of getting into print now dream of seeing their story on the big or small screen."
Who is a screenplay writer called in short?
A screenplay writer (also called screenwriter for short)