For example, I could jump on the treadmill and jog at 7km/h for 1min 30secs then go into a flat out run at 18km/h for the next 30 secs - totaling 2 mins - and repeat this 8 times. After spending a minute or two cooling down this would complete a nearly 20 minute high intensity cardio session that offers a change in routine, a faster workout, and the potential to burn 200-300 calories in one go - that's half your dinner!
Research around 'HIIT' suggests that a 9-20 minute session is actually far more effective than a 40-60 min continuous workout (running at the same pace) - so if you are always strapped for time this is for you! It is proven to increase something called Excess Post-Exercise Oxygen Consumption (EPOC), which basically means you are still burning calories from this workout hours after you have left the gym - what more could you want from a training method? Try your hand at it on your next trip to the gym!