A powder room is usually a room with a toilet and sink. It is also called a "half bath". It does not have a shower or bathtub. Often it is placed on the main hall or near the "public" rooms of a home on the ground floor and is used by the day guests.
At least in the U.S., real estate descriptions count this as a half in the count of bathrooms in a home. So, 2.5 bathrooms will have 2 full bathrooms (with toilet, bath, shower and sink facilities) and a powder room. A less common term is a three-quarter or .75 bath with a shower or bath, but not the other.