Interval training involves bursts of high intensity work alternated with periods of low intensity activity. Coaches use intervals because they improve cardiovascular fitness and increase areobic capacity making faster fitter well-rounded runners.
Interval Training is also a great way to boost metabolism and burn fat.
If you are new to intervals try Pyramid 180 MAX. After a 5 minute warm up you get 12 minutes of short sprints and recoveries. If you are wanting to increase your speed 3 Step intervals gives longer sprints of 3 minutes at 160, 170 and 180 BPM each with a 3 minute recovery.