Bathroom: Any room with a toilet in it.
Restroom: Any such room in a place that's charging you money somehow.
Washroom: same as bathroom, but used by people who want to sound more polite.
Powder Room: same as washroom, but for ladies. From the excuse "I must go powder my nose."
Lavatory: Like washroom/powder room, but for people who either belong to a country club or want you to think they do.
