I have received a lot of questions regarding this topic as of late, soooo here comes some sciencey stuff!
Aerobic can be defined as INVOLVING oxygen, whereas anaerobic can be defined as NOT INVOLVING oxygen. If we remember from a past post regarding cellular energy production, we burn different amounts of pyruvate and lactate depending upon how much oxygen is present in our cells at the time of glycolysis.
Aerobic exercise can be defined as INVOLVING oxygen. The primary fuel for this type of exercise is pyruvate.
If oxygen is plentiful at the time of glycolysis, our cells will produce high levels of pyruvate and low levels of lactate to be used by the mitochondria for ATP production. This type of energy production is extremely efficient and can be repeated literally all day long as long as you are fueling properly during exercise, hydrating, and have enough chamois cream :-). This is aerobic exercise.
Anaerobic exercise can be defined as NOT INVOLVING oxygen. The primary fuel for this type of exercise is lactate.
If oxygen is not plentiful at the time of glycolysis, our cells will begin to produce increased lactate. The ratio of lactate to pyruvate produced will continue to change as the intensity becomes higher and/or oxygen levels decrease. Eventually, you hit your lactate threshold which is the level at which your body cannot process the lactate being produced effectively and it begins to accumulate rapidly in the bloodstream. As the body tries to utilize more and more lactate, hydrogen is produced as a byproduct. The issue here is hydrogen lowers the pH of the blood present in the muscle (makes it more acidic) which is why your muscles burn with high intensity exercise (acidosis). As the pH of the muscle continues to lower and become more acidic, muscular contractions will become less forceful and eventually cease if the muscle pH becomes low enough. This is anaerobic exercise.
One of the most interesting aspects of cycling is it involves both aerobic and anaerobic efforts at times. So, when you are training for your goal event or race, be sure to include both long steady efforts to improve aerobic capacity, but also short and hard efforts (with plenty of rest between) to improve anaerobic capacity.