In everyday speech, we tend to use these two terms interchangeably. However, restrooms and bathrooms are two different things, and the terms are actually not interchangeable.
Restroom
A restroom is a public facility that usually includes several toilets and sinks to accommodate the simultaneous use of multiple people. They are usually found in public and commercial spaces, such as stores, malls, restaurants, clubs, office buildings, and hotel lobbies. There is no privacy in a restroom, and the common expectation is that individuals will use the facilities quickly and exit to allow the next person to use it.
Bathroom
A bathroom is a private facility that typically includes a bathtub and/or shower, one toilet, and one sink for the use of a single person. More modern homes will sometimes have two sinks, typically stated to be his and hers, in the master bathroom. Privacy is expected in a bathroom, and there are no expected time constraints on use. Bathroom decor is typically more decorative, as bathrooms are typically found in homes and private hotel rooms.
Privacy and location are the determining factors between restrooms and bathrooms. These two types of facilities are not to be mistaken for each other, and the terms should not be used interchangeably.