Mode of interaction between two species in which one species (parasite) depends on the other species (host) for food and shelter/one organism is benefitted, the other is harmed. 

e.g. Human liver fluke/Malarial parasite/Cuscuta.

Co-evolution is the relationship between two interacting organisms where both organisms failed to survive in the absence of the other. 

e.g. Fig and Fig wasp/ophrys and bumble bee.

Detailed Answer :

Parasitism it is the interaction between two species in which one species, called parasite depends for food & shelter on the other species called host and in this Process one species (parasite) is benefited while the other-the host is harmed or damaged e. g. tick & dog, fig. & fig & fig wasp.

Coevolution : When evolutionary changes is are species prompt the corresponding changes in other species with which the former interact, the phenomenon is called co-evolution, e,g host & pathogen, brey & predator etc. The parasites usually get host specific in such a way that both the hast & pathogen tend to co-evolve i.e. if the host develops the mechanism for resisting the parasite for its protection the pathogen also evolve the mechanism to counteract & neutralize them to succeed with the host species.