A restroom is a public bathroom or lavatory. You might stop at a fast food restaurant during a long drive to use the restroom and buy some fries.
You'll find restrooms in restaurants, libraries, schools, highway rest areas, and shopping malls, among other places. Many restrooms are open to the public at large, while others are reserved for customers. While the term restroom is extremely common in the U.S., a restroom in Britain doesn't usually contain a toilet — it is simply a place to sit and rest. And in Canada, it's more often called a washroom.