Spiders, for example, or lobsters, have tough outer shell systems which provide rigidity and shape to their bodies.
The types of animal that have exoskeleton are mollusks and arthropods. Exoskeleton gives them a rigid support, protection. In exchange, the exoskeleton will interfere with the growth of the animal. To overcome this, arthropods goes to a process call molting. Through this process, they basically shed their exoskeleton and replace it with a new, larger one.
Medieval armour (in the case of mounted knights) furnishes an example of an artificial human exoskeleton. Modern motorists use automobiles as temporary protective exoskeletons in harsh traffic environments.
Excellent as a principle of defence, exoskeletons may neverthess cause problems where entities carry an excessive weight to surface-area ratio; or whenever organism growth requires an enlarged exoskeleton.