The terminology isn't fixed, but in my book, a traffic circle is where traffic on the rotary has to yield to traffic entering, whereas traffic on a roundabout has priority over traffic entering.

Roundabouts are typically smaller (down to a mini-roundabout with an island just a metre or two across, compared with traffic circles which can be hundreds of metres in diameter). As a result, roundabout traffic typically moves slower, and they're MUCH safer.