Aerobic Interval Training:

This is a hybrid of the two previous form of training. It's mostly used with beginners who have trouble performing 20 minutes of continuous activity (as outlines by "Aerobic Training").
Aerobic Interval Training is done by performing 5-10 minutes of low intensity aerobic work, then taking a short break, then doing another 5 minutes of low intensity aerobic training, until the participant is fatigued. Each time it's done, try to increase the amount of time you run/bike/elliptical before you take a break, e.g. Week 1 = 5 minutes segments, Week 2 = 7 minute segments, etc.
Related Interval Training Articles:
What Is The Best Interval Training Program? - By Workout Of The Week
Interval Training: Sport-Specific Conditioning. - By Craig Ballantyne
Interval Training 101. - By Peak Performance
Other Interval Training Articles...
The Benefits Of This Protocol Are:
It's excellent for untrained/beginner individuals and it helps build aerobic capacity.
The Disadvantages Of This Type Of Training Are:
The disadvantages of this type of training are almost the same as the ones for aerobic training: It can get boring, you can develop overuse injuries, and sometimes endurance activities cause muscle loss.