They are often used to mean the same thing in American English. If one is being particular about their word choice, a restroom typically has a toilet and sink (like in a restaurant or some truck stops) and a bathroom has a toilet, sink, and bathtub or shower (like in most homes or some truck stops). The following sentence maddeningly makes sense: You can only use the bathroom in a restroom, but you can use the bathroom and bathe in a bathroom.

IN THE UNITED STATES: "Restroom," "bathroom," "toilet:" --"Restroom" is the most polite and euphemistic --"Bathroom" is polite, but direct --"Toilet" is very direct, and often refers to the porcelain fixture itself In a strange place I would probably ask "where's the men's room" or "where are the restrooms?" In a friend's house I would ask "where's the bathroom?" In my own house, talking to my wife, I would call it "the toilet."

The British don't tend to use the word "restroom".Bathroom is the room where you wash and take a bath.If you want the other room you have to say "toilet". IN the US a bathroom is a British toilet.