Circuit was originally a workout where you move quickly from one weight exercise to another for a different body part (with weight machines set up in a circuit), but now often includes cardio and weights. Cathe's circuit workouts are usually weights and cardio workouts. Tony Horton's weight workouts are often set up circuit style.
Interval is interspersing short periods of high intensity (near anaerobic level---where you can't get enough oxygen to continue for very long) with steady state cardio. The IMAX workouts are an example of this.
Hi/low stands for 'high"(impact) and "low"(impact) mixed in the same cardio workout.
HTH