I'm not Cathe, but . . .
In an interval workout, you alternate among moderate- to mid-range intensity of a given cardio mode (say power-walking and jogging) with high-intensity "sprints" or "blasts" of similar mode (say running), then do a bit of a recovery and start the whole thing all over again. Walk-jog-SPRINT, walk-jog-SPRINT. (Just an example.) The sprints or "blasts" are intended to push you way high into your target heart rate, perhaps up to the anaerobic threshold, and interval training is proven to be a very time-efficient way to increase cardiovascular endurance.
In a circuit workout, you alternate segments of unweighted cardio work (say step aerobics) with weighted resistance cycles (say a minute or two of barbell squats), keeping in mind that the main focus of these resistance cycles is to maintain an elevated heart rate rather than promote strength in the muscles. (You do, however, enjoy muscle conditioning benefits with circuit!) Also a very effective training mode.
A-jock
A-jock