Parasitism is a type of symbiotic relationship between organisms of different species where one organism, the parasite, benefits at the expense of the host.

In general, parasites are much smaller than their host, show a high degree of specialization for their mode of life, and reproduce more quickly and in greater numbers than their hosts.

Classic examples of parasitism include interactions between vertebrate hosts and diverse animals such as tapeworms, flukes, the Plasmodium species, and fleas.

Parasitism is differentiated from parasitoidism, a relationship in which the host is always killed by the parasite such as moths, butterflies, ants, flies and others.

Parasites increase their fitness by exploiting hosts for food, habitat and dispersal.

(taken from Wikipedia)