Respiration is not breathing. Respiration is the breakdown of glucose (sugar) to produce energy. There are two different types of respiration: aerobic and anaerobic. The difference between the two is that anaerobic does not involve oxygen and aerobic does.                                          

Aerobic respiration (with oxygen) is far more effective in releasing energy than anaerobic.

When you are sitting relaxing, you will be respiring aerobically but after you have been running for a while you will start to respire aerobically. This is why you get tired when you run for a long time – anaerobic respiration produces less energy.