Coelom is the body cavity which is lined by a mesoderm. The presence or absence of body cavity or coelom plays a very important role in the classification of animals.
Animals that having  a cavity between body wall and digestive tract are known as coelomates for eg. annelids, molluscs, arthropods, echinodermates, and chordates etc.
The animals in which the body cavity is not lined by mesoderm are known as pseudocoelomates. In such animals, mesoderm is scattered in between ectoderm and endoderm. Aschelminthes is an example of pseudocoelomates.
Animals in which the body cavity is absent  are known as acoelomates For e.g platyhelminthes.