What are the three types of Energy Systems?

ATP-PC System (Phosphogen System) - This system is used only for very short durations of up to 10 seconds. The ATP-PC system neither uses oxygen nor produces lactic acid and is thus said to be alactic anaerobic. For example a 100m sprinter would use this energy system.
Anaerobic System (Lactic Acid System) - Predominates in supplying energy for exercises lasting less than 2 min. Also known as the Gylcolytic System. An example of an activity of the intensity and duration that this system works under would be a 400m sprint.
Aerobic System - This is the long duration energy system. By 5 min of exercise the O2 system is clearly the dominant system. For example A long distance or marathon runner.

ATP-PC System

ATP-PC System - 'ATP-PC system' stands for 'Adenosine-Triphosphate - Phosphocreatine system'. This energy system is used as a rapid way to regenerate ATP in the body, and typically lasts for 10 seconds. This is importants as if you used all your stores of ATP in the body in one go, they would be depleated in roughly 3 seconds. It is only able to last for a short duration longer than that of normal ATP, as 1 PC molecule can reform only 1 molecule of ATP, once all the PC in the muscle cell is broken down, this energy system no longer functions, and the lactic acid system takes over. The ATP-PC system allows high intensity activity such as very explosive movements to last longer.
The reason it is described as ATP-PC is that these two compounds are part of a coupled reaction; where the energy released from one reaction is used to restore the energy in the other. The breakdown of ATP into ADP and an inorganic Phosphate is an exothermic reaction, which gives off energy. The breakdown of Phosphocreatine (PC) to creatine and an inorganic phosphate molecule is also exothermic. The energy released from the breakdown of PC is used in an endothermic reaction to rejuvinate the ATP molecule. The enzyme that is active during the rejuvination of ATP from ADP and the inorganic Phosphate molecule is ATPase. This all occurs in the sarcoplasm of muscle cells and is said to be anaerobic as oxygen is not required for it to work. When the intensity of exercise drops, Phosphocreatine can be reformed by the enzyme Creatine Kinase.

Anaerobic System or Lactic Acid System

Anaerobic-In the absence of, not requiring, nor utilizing oxygen. This energy system works by converting the pyruvate produced from the glycolysis of glucose, into lactate, converted by the enzyme Lactate Dehydrogenase (LDH). The purpose of this system is to regenerate 2 ATP molecules for every 1 molecule of glucose that is used. The mechanisms of this system involve the oxidising of NADH + H+, back to NAD+ by producing the aforementioned lactate. This is so the NAD+ can be re-used in the conversion of glucose to pyruvate to produce 2 ATP molecules. The ATP regenerated allows, in the human muscle, exercise of higher intensity to continue up to the point that the lactate reaches toxic levels, at which point the muscle will start to fatigue and eventually stop working all together.

Aerobic System

Aerobic -In the presence of, requiring, or utilizing oxygen. The purpose of this system of producing energy is to produce 38 molecules of ATP from each molecule of Glucose that is used. ATP is broken down in the body to give ADP and an inorganic phosphate plus energy. This energy system is used throughout the body, for producing energy for all for metabolic processes, in exercise it is used for sub-maximal exercise such as long distance running. This system is used to regenerate the ATP that is used for energy in the body.
This energy system is otherwise known as 'aerobic respiration' and can be split into 3 separate stages: Glycolysis, The Krebs Cycle and Oxydative Phosphorylation.
Glycolysis - The first stage is known as glycolysis, which produces 2 ATP molecules, a reduced molecule of NAD (NADH), and 2 pyruvate molecules which move on to the next stage - the krebs cycle. Glycolysis takes place in the cytoplasm of normal body cells, or the sarcoplasm of muscle cells.
The Krebs Cycle - This is the second stage, and the products of this stage of the aerobic system are a net production of 1 ATP, 1 Carbon Dioxide Molecule, three reduced NAD molecules, 1 reduced FAD molecule (The molecules of NAD and FAD mentioned here are electron carriers, and if they are said to be reduced, this means that they have had a H+ ion added to them). The things produced here are for each turn of the Krebs Cycle. The krebs cycle turns twice for each molecule of glucose that passes through the aerobic system - as 2 pyruvate molecules enter the Krebs Cycle. In order for the Pyruvate molecules to enter the krebs cycle they must be converted to Acetyl Coenzyme A. During this link reaction, for each molecule of pyruvat that gets converted to Acetyl Coenzyme A, an NAD is also reduced. This stage of the aerobic system takes place in the Matrix of cells Mitochondria.
Oxydative Phosphorylation - This is the last stage of the aerobic system and produces the largest yield of ATP out of all the stages - a total of 34 ATP molecules. It is called 'Oxydative Phosphorylation' because oxygen is the final acceptor of the electrons and hydrogen ions that leave this stage of aerobic respiration (hence oxydative) and ADP gets phosphorylated (an extra phosphate gets added) to form ATP (hence phosphorylation).
This stage of the aerobic system occurs on the cristae (infoldings on the membrane of the mitochondria). The NADH+ from glycolysis and the krebs cycle, and the FADH+ from the krebs cycle pass down electron carriers which are at decreasing energy levels, in which energy is released to reform ATP. Each NADH+ that passes down this electron transport chain provides enough energy for 3 molecules of ATP and each molecule, and each molecule of FADH+ provides enough energy for 2 molecules of ATP. If you do your maths this means that 10 total NADH+ molecules allow the rejuvination of 30 ATP, and 2 FADH+ molecules allow for 4 ATP molecules to be rejuvinated (The total being 34 from oxydative phosphorylation, plus the 4 from the previous 2 stages meaning a total of 38 ATP being produced during the aerobic system). The NADH+ and FADH+ get oxidized to allow the NAD and FAD to return to be used in the aerobic system again, and electrons and hydrogen ions are accepted by oxygen to produce water, a harmless by-product.

Online Activity
NSW HSC Energy Systems Online Worksheet - http://hsc.csu.edu.au/pdhpe/core2/performance/4-1/4.1.1Energy_Systems.html
Energy Supply for Muscles Website - http://www.nismat.org/physcor/energy_supply.html#Top

Other Useful Links:
James Madison University - Strength and Conditioning Program - excellent information

Clickview Resources:
All Systems Go Energy Systems of the Body - 30 minutes

All Systems Go Again - 30 minutes