A parasite is an organism that lives in or on the living tissue of a host organism at the expense of it. The biological interaction between the host and the parasite is called parasitism. Parasitism is a type of symbiosis, by one definition, although another definition of symbiosis excludes parasitism, since it requires that the host benefit from the interaction as well as the parasite.
Parasites are generally smaller than their hosts, absorbing nutrients from the host's body fluids, but this is far from a universal strategy. Organisms whose life cycle guarantees the death of the host are not called parasites, but are parasitoids. A few parasites have hosts which are themselves parasites. These are called hyperparasites.
Endoparasites (endo = within; parasites that live inside their hosts)
Ectoparasites (ecto = outside; parasites that live on but not within their hosts, for example, attached to their skin)
Many lifeforms are parasitic only during a part of their lifecycle. Many cuckoos, for example, are brood parasites: their young are parasitic on the host species, but adult cuckoos fend for themselves. (A species' own offspring does not qualify, of course, as that would necessarily mean classifying every organism to exist as a parasite of its parent host.)
The term parasite is also used for people who benefit from a relationship, society or system without contributing significantly to it, although they could; however this usage of the term is highly controversial because of its association with Nazism and its various extermination programs.