There are probably as many different versions of interval training as there are runners. There are short and long intervals, but what is the differences between these two and what to take account.

Long intervals also known as intervals or hard training. Intervals are often measured either time or distance, for example 5 x 2min or 5 x 800 meters. The minimum time that one interval takes is often 2 minutes but it normally vary between 2-4 minutes. In distances it’s normally set between 800m – 1500m.
If intervals are doing in distance, pace depends runners fitness level, but it should be the pace that runner could maintain during all sets. Recovery times are often one to two minutes, even 3-4 if distance is longer, but it should never last longer than actual running. It also good to jog recovery times.
Intervals increase available energy by increasing mitochondrial level.

Repetitions also known as short intervals. Repetitions are shorter than intervals. For the long distance runner they are often 200-800 meters long, but repetitions should never take more than 2 minutes, which means the slower runner can’t run the longer distances like 500m. It’s more important to think efforts that are put to training session than the covered distance. Number of set depends on runners fitness level and weekly mileage, as those how running less and are not as fast spent more time to cover the distance which increases the whole amount of the time that is spent the session. Recovery time between each repetition differ a lot from interval, when intervals is always shorter than the actual run, in repetition recovery time is two to three times longer than actual run. Recovery time is longer because each run are faster than interval pace and body need more time to recover before the next run.
Repetitions improve speed, running economy and anaerobic power.

How much you should include these your training? It all depends on which session you are now. But the recommendation is that you should not spend more than 8% or 10km (which ever come first) by doing intervals during the week and no more than 5% or 8km (which ever come first) per week.