Aerobic and anaerobic respiration are two ways by which cells get energy from food, mainly glucose.
Aerobic respiration needs oxygen to take place. It happens inside the mitochondria. In this process, glucose reacts with oxygen to produce carbon dioxide, water, and a large amount of energy in the form of ATP. This is the main way most living organisms, including humans, get energy.
Anaerobic respiration happens without oxygen. It occurs in the cytoplasm of cells. In this process, glucose is broken down only partly, so it produces much less energy. The end products are different — in animals, it forms lactic acid, while in yeast and some bacteria, it produces alcohol and carbon dioxide.
In short, aerobic respiration gives more energy and cleaner byproducts, while anaerobic respiration is faster but produces less energy and can lead to the buildup of harmful substances like lactic acid.