This is an important distinction, because the older traffic circles aren't always easy to navigate, so they haven't been very popular. In older traffic circles, no physical structures prevent drivers from speeding right into the intersection. Another feature is that vehicles approaching roundabouts yield to circulating traffic. No stopping is required. Some older traffic circles and many conventional intersections alternate traffic with stop signs or signals.
Roundabouts enable all cars to move continuously through intersections at the same low speed.
People assume that because there are so many traffic signals out there, they must be efficient. The fact is, they're not. When half of the cars are stopped at an intersection at any given time, delays are inevitable. Other design elements set roundabouts apart from traffic circles. Pedestrians cross only at the perimeter, vehicles can't turn left ahead of the central island, and parking isn't allowed inside the circle.