A journalist is a person who practices journalism.
Many people consider journalist interchangeable with reporter, a person who gathers information and creates a written report, or story.
However, this overlooks many other types of journalists, including columnists, leader writers, photographers, editorial designers, and sub editors (British) or copy editors (American).
I've noticed recently that people like Armstrong Williams have been "elevated" to the level of journalist. I suppose it's a convenient label.