A decorator is someone whose job is to paint rooms and put paper on the walls. In British English this is a very ordinary job-word: you phone a decorator when the house looks tired. They may also fill cracks and help choose colours, but the core work is paint and wallpaper, not designing the whole house from nothing. Though actually, American English often says painter, or painter and decorator, for that same person. Interior decorator in American talk is closer to someone who picks furniture, fabrics, and a style — a different, often more expensive-sounding job, nearer to interior designer. 'Decorator' by itself can sound a little older next to 'interior designer,' which people use when they want the work to sound high-end. The person who paints your bedroom in the British sense is still a decorator, even if they never choose a sofa.