All of the living cells in your body need energy. Normally the body converts glucose into energy using oxygen. This process is called aerobic respiration.
Oxygen is taken into your body by the lungs and carroed around the body by your blood.
During physical activity, your muscles are working harder and so need more energy than normal. That means they need to be supplied with more blood and oxygen to release the energy.
Activities where your body can keep up with oxygen demand are called aerobic activities.
Aerobic activities are any type of exercise that is done at a steady rate, and slow enough so that your heart can keep your muscles supplied with the oxygen they need.
If your body can't supply the oxygen needed, your muscles release energy without using oxygen.
Activities where your body has to do this are called anaerobic activities. An anaerobic activity is any exercise done in short, fast bursts - where your heart can't keep up with your muscles' need for blood and oxygen.
Anaerobic respiration releases energy, but also produces a mild poison called lactic acid.
Lactic acid build-up soon makes your muscles feel tired - so this is used for short, strenuous activities. This is how sprinters get their energy.
You need oxygen to get rid of this lactic acid once you've stopped exercising.