Fartlek vs. Intervals

Fartlek is Swedish for "speed play." These are random bursts of speed of random length/duration. It's a fun way of doing some fast running without structure. I used to do it by saying, "I'm going to run fast for the next three telephone poles" or something like that. These runs help train your body to run fast, but without the pressure of running certain distances, number of reps, or times.
Intervals are usually shorter distance fast repetitions, often done on a track. An example would be to run 6x800m with 3 minute recoveries. This means you run 800m fast (at the pace you might run a 5k or faster), then jog 3 minutes really easy to recover. Repeat a total of 6 times. Standard interval distances are 200m, 400m, 800m, 1000m, and 1600m. These are "red-line" type workouts which force your body to run under duress. Interval training increases your overall speed, your VO2max (the maximum amount of oxygen your system can uptake -- more is better), but are usually strenuous enough to not be undertaken more than twice a week, and only then after you've developed a mileage base. These are the tune-up you do in the 8~12 weeks prior to a big race, not a year-round type workouts. The longer your target race, typically the longer the interval. Most interval workouts are done at 5k pace or faster.I should note that intervals do not have to always be distance based, nor do all intervals have to be the same length. Examples would be 5 x 3 minutes fast with 3 minute recoveries, or a time based interval like this one, which is one of my favorites.

Source: http://www.runningahead.com

Understanding for Referee's Cooper Test:
If my target is to run each 400 meters in 1:20 secs. My interval training bout shall be 8 x 400 m at 1:15 secs with a resting interval of 3 minutes in between repetitions or 6 x 800 m at 2:30/2:35 secs with a resting period of 5 minutes. Training must only starts after proper warm ups and for this target your basic fitness level should be 2800 m in 12 minutes, otherwise try not to be too ambitiuos. Build up your target gradually.