The problem is that it is
The problem is that it is almost impossible to get correct if you don't get it tested. I agree with blind boy with the exception of it being about lactate threshold. It's more about caloric usage in terms of which calories are you drawing from the most at each range of heart beats.
This information will be very watered down but the basics are this: the body has two basic forms of energy; those being fats and carbs. Based on your oxygen intake abilities or VO2 max you will burn more of one or the other depending on your level of exertion. We use heart rate as the scale to measure this exertion. As you work, your body will utilize your calories to fulfill its needs for things like muscular contractions, organ function, etc. Fats take more effort for the body to use because they must be turned into a usable energy as the body cannot just take fat as fat and burn it. It must mix it with oxygen and put it through a 12-15 stage (the stages are constantly debated) process to convert it into a usable energy source (basically sugar). This is very difficult for the body to do at higher levels of exertion because the oxygen is less available so the body uses sugar which needs no oxygen to be burned as fuel.
It breaks down something like this. Lets say you can bring in 30ml of oxygen per minute at rest and 40ml at peak. You have a resting heart rate of 90 and use only 10ml of your available oxygen for body function leaving you with 20ml for fat conversion. At peak you have a heart rate of 180 and use 38ml of oxygen for body function leaving you with 2ml for fat conversion. At rest you have a larger level of oxygen not being used for basic function so you can allocate that left over oxygen to the process of converting fat and utilize a higher amount of fat calories for your energy. At peak you have very little oxygen left over so you will be burning nearly 100% carbs to keep going. Somewhere in between resting and peak you will hit a "wall" and the brain will recognize that you can no longer maintain your level of exertion by using fat because you just don't have enough oxygen available for that purpose so it switches to carbs for energy. This is commonly referred to as your Anaerobic Threshold or AT. Keep in mind at all levels of exertion you will always be burning fats AND carbs but the amount of oxygen available will determine the percentage of each.
Base training is considered to be done below your AT so you can teach your body to use fat as an energy source which will build your overall endurance because you can run much much longer on your fat stores than you can your carbs. For the person above they might have an AT of say 150 so training in intervals below 150 they will be keeping their body efficient and build their ability to burn higher percentages of fat. Above the 150 mark they will use carbs for energy, last for a shorter time, and that is where the ideas and issues of lactate threshold really come into play. As you work in your base area you will raise the level of fats you convert and that will allow you to run faster with less energy and then doing higher intensity intervals above your AT will build on lung capacity and muscular power. Blending of the two will help you raise your AT and allow you to begin burning higher amounts of fat at higher heart rates and you will then gain more speed with less or equal efforts while still maintaining the ability to continue for long periods.
The problem is that you have to be tested through gas exchange testing in order to have the numbers be accurate. The generic and often used equation to determine your AT without such testing is to subtract your age from 180 and that is your AT. Add 10 if you are in great shape and subtract 10 if you are in poor shape. Take that number and split it into 5 zones that consist of a 10 beat range. Zone one being your warm up and cool down range, Zone 2 your base range, Zone 3 which ends at AT being your endurance range, Zone 4 being a performance range, and Zone 5 your all out peak range. The scale would look like this for a person with an AT of 150 - Z1: Resting-130, Z2:130-140, Z3: 140-150, AT, Z4:150-160, Z5: 160-Peak
Any questions?