The following is a pretty geeky, yet, a simple summary of the energy systems in the human body. If you have any background in biology, chemistry, or physiology you'll probably be alright.
To sum up the summary: There are three main pathways which provide energy for the human body. The first, phosphocreatine provides rapid energy production when a quick burst is needed. For example, during a reflex, when having to dodge traffic, running to catch the bus, or performing 1 - 5 reps during weightlifting. Glycolysis is simply using glucose(sugar) to create energy. This process can be done with (aerobic) or without (anaerobic) the use of oxygen, and the difference is in the energy yield. Our bodies want to utilize oxygen as much as possible. This is a major reason for exercising to increase your aerobic capacity (how well your body uses oxygen). This takes time however, and if the intensity of exercise is too high, much more energy will be formed without oxygen. Finally, aerobic metabolism can be completed without the use of glucose. In this instance fat is broken down and used as fuel, or protein is converted into glucose and then used as fuel.
Bottom line: When you exercise, you are always utilizing all three energy systems and your body will always try to use oxygen to the fullest to meet its energy demand. The overload principle can be applied to cardiovascular exercise as well, and if you push yourself out of your comfort zone your body will adapt and improve.
Now for you science geeks
When maximal force is exerted by the muscle tissue, the minimal amounts of ATP readily available in the muscle are broken down to ADP. As the ratio of ADP to ATP rises phosphocreatine is called to action. When this compound decomposes a huge amount of energy is released, ~10,300 calories per mole. Since this is much more than the amount of energy contained within the bond of ATP rapid synthesis of ATP from ADP occurs. The human body stores enough phosphocreatine to provide maximal muscle power for ~8 to 10 seconds.
When the need for ATP is high, and phosphocreatine has been exhausted, there is a need to increase the rate of glycolytic activity. Hydrolysis of glucose during the initial steps; ends with the formation of pyruvate. The formation of pyruvate yields 2 ATP, a very small amount, which is why glycolytic activity can increase thousands of times above resting values. As ATP is split at faster rates, excess hydrogen is released into the cytoplasm. NAD⁺ is an important coenzyme in glycolysis; it keeps the intracellular Ph in balance. In order to recycle NAD⁺ quickly, a majority of the pyruvate combines with NADH forming lactate + NAD⁺. Lactate can accept additional H⁺ ions; in this role lactate aids the intercellular buffering system and protects the cell from acidosis. With this additional H⁺ molecule it is now referred to as lactic acid. If the concentration of lactic acid inside the cell is greater than that outside the cell, lactic acid will escape through the cell membrane via osmosis. This is perhaps where the association of lactic acid accumulation got its bad reputation because it is indeed responsible for transferring H⁺ out of the cell. However, it should be recognized that lactate is only the acceptor of the hydrogen ion; the source remains hydrolysis of ATP during the initial breakdown of glucose. After lactate is formed it can be utilized either as an intracellular buffer, as a source of ATP production, or transported to the liver and kidneys for regeneration of glucose. Since oxygen is not present in these reactions to create energy, it is referred to as anaerobic, or without oxygen.
When the need for ATP is low, glycolytic pathways predominantly send their pyruvate molecules to the mitochondria to complete citric cycle. This is ideal because it yields the highest amount of ATP (36-40) per molecule of glucose. Reactions take place in the inner membrane of the mitochondria, and one ATP is produced. Hydrogen ions are also released and picked up by NAD⁺ and FAD forming NADH and FADH₂. Through a system of proteins gates, NADH and FADH₂ release the hydrogen into the outer compartment of the mitochondria forming an energy gradient. This is known as the electron transport chain. When the energy gradient reaches a threshold, hydrogen rushes back into the inner membrane through a special protein gate, which is high in an enzyme called ATPase. ATPase uses the energy from the hydrogen ions to combine ADP and phosphate ions; creating ATP. After diffusing back into the inner membrane hydrogen ions combine with oxygen to form water, hence the name aerobic, which means the presence of oxygen. Through this same cycle, the mitochondria can utilize a byproduct of fat breakdown called acetylCOA. The same number of ATP is produced when acetylCOA completes the citric acid cycle. Therefore, this pathway can spare glucose and produce an adequate energy supply under most conditions.