What is speed interval or Sprint Interval?

In Interval Training, the set or interval in which you induce greater exertion is called as the speed interval or the sprint interval. This interval lasts for 15-60 seconds depending on your capacity.

What is rest Interval?

The rest interval is the set alternating with each sprint or speed interval, where you dial down on your level of exertion. The idea is to help recover from the short high burst set to facilitate yet another such set.
 
Mind you, the set is called rest interval but it does not imply that you stop all activities and take a break. You must simply lower the intensity of your exercise and do it at a slower pace to facilitate a recovery of your cardiovascular systems.

For Example: Let’s consider your choice of exercise as running.

The workout would begin with you running at a speed of 12 km/h or 7.45 mi/hr. This, let’s say is your speed interval and you hold the speed for about 60 seconds. Now, you’d immediately bring the speed down to about 6-7 km/hr or 3.75-4.375 mi/hr. This would be your rest interval and this would last for 60-90 seconds depending upon your capacity. If you’re capacity is still developing then a rest interval of 120 seconds is advisable.

But keep in mind that interval training is not for beginners who are beginning an exercise program or picking up after a long break. The general suggestion is that you begin with light intensity cardiovascular workouts building gradually to higher intensities and eventually dwelling into interval training as a program staple.

Time

The time period per workout for an interval training session need not last beyond twenty minutes. Tops. However at many advanced levels even twenty minutes are a far stretch. So roughly 9-20 minutes of workout, warm-ups and cool downs excluded, would be ideal.

Intensity

Intensity of an interval is how hard you push yourself during a set or interval. To keep things simple intensity is usually measured on a scale of 1 to 10, with 1 being absolutely no effort and 10 being the maximum effort possible.
Now the markings on this scale are thoroughly subjective and depend on your level of training experience. Let’s consider a person who hasn’t trained for years and starts today by walking briskly and this leaves him out of breath and all of it, within a matter of a minute. This would be their 10.
But on the other if a person has a few years of good training behind them, then their 10 would actually be a flat out sprint for, say, 20 seconds to be left out of breath.
Gauging by this example you may determine your levels and begin training. As a beginner of decent health and cardiovascular and muscular capacity, intensity intervals of 4-6 are advisable. As you progress you’d want to raise the bar to intensity intervals of 7-10.

Train hard and train safe.