It is an association whereone organism, the parasite, causes harm to another, the host.
Examples of tapeworms, fleas, barnacles, etc.
Mutualism:
It is a relationship between the host and a symbiont, where both organisms benefit and no one is harmed.
Example- corals & algae, humans & bacteria, etc.
Mutualism can metamorphose into parasitism.
For example- the oxpeckers are mutualists with the antelopes when removing ectoparasites. However, they become harmful to the antelopes when they peck their scabs and feed on their blood.