The term coevolution is used to describe cases where two (or more) species reciprocally affect each other's evolution. So for example, an evolutionary change in the morphology of a plant, might affect the morphology of an herbivore that eats the plant, which in turn might affect the evolution of the plant, which might affect the evolution of the herbivore...and so on.
Coevolution is likely to happen when different species have close ecological interactions with one another. These ecological relationships include:
Plants and insects represent a classic case of coevolution - one that is often, but not always, mutualistic. Many plants and their pollinators are so reliant on one another and their relationships are so exclusive that biologists have good reason to think that the "match" between the two is the result of a coevolutionary process.
But we can see exclusive "matches" between plants and insects even when pollination is not involved. Some Central American Acacia species have hollow thorns and pores at the bases of their leaves that secrete nectar (see image at right). These hollow thorns are the exclusive nest-site of some species of ant that drink the nectar. But the ants are not just taking advantage of the plant - they also defend their acacia plant against herbivores.
This system is probably the product of coevolution: the plants would not have evolved hollow thorns or nectar pores unless their evolution had been affected by the ants, and the ants would not have evolved herbivore defense behaviors unless their evolution had been affected by the plants.