Supersetting is a method of training utilised by individuals seeking a broad range of goals. It involves completing a set of one exercise and immediately moving into a second, without taking a break.
The general principle is to use two exercises that are functionally different to one another each other. When performing supersets, it is common to combine a compound exercise with an isolation exercise.
The role of supersetting is to maximise muscle recruit during a set, add intensity through training beyond fatigue and to burn more calories and increase your heart rate.
An example of a superset when training quadriceps might be to perform squats as your compound exercise, and then move onto the leg extension as your isolation exercise. Alternatively when training Chest, you might superset a Bench Press as your compound exercise with Dumbbell Flyes as your isolation exercise. The key is to make sure the exercises are fundamentally different in their movement.
A poor example of a superset would be a Flat Barbell Bench Press supersetted with an Incline Barbell Bench Press. The exercises are too similar and will not stimulate additional muscle fibres that are not already recruited. Similarly, if you are using two isolation exercises such as Flat Dumbbell Ffyes supersetted with a Decline Dumbbell Fly, as the movements are too similar to illicit the extra benefit that you are trying to achieve.