A lot of people seem to mix up the two terms (even instructors).
Interval workouts are cardio workouts that include higher-intensity intervals that are near anaerobic levels ("blasts," like the IMAXes).
Circuit workouts started in gyms, where they had people going from weight machine to weight machine with no break in between, to get a fast, efficient weight workouts.
Some places put cardio "stations" in with the weight machines to make a "cardio and weight" circuit. (Like "Curves" franchises do).
In general, circuit means going from one exercise to another without a break (or with a very short break). It can be all weights, cardio and weights, or even all cardio, I suppose (10 minutes on a bike, followed by 10 minutes of rope skipping, then 10 minutes on the treadmill, etc.). You can repeat the circuit more than once during a workout.