Each party in coevolution relationship exerts selective pressures on the other thereby affecting each other's evolution. It is likely to happen when different species have close ecological interactions with one another.
Coevolution influences the structure and function of ecological communities as well as the dynamics of infectious disease. An evolutionary change in the morphology of a plant, might effect the morphology of a herbivore that eats the plant.
Some south central American Acacia species have hollow thorns and pores at the bases of their leaves that secrete nectar. These hollow thorns are the exclusive nesting sites of certain species of ants that also consume the nectar. In turn these ants defend the Acacia plant against herbivores. Thus the plants would not have evolved hollow thorns or nectar pores unless their evolution had been affected by the ants. Similarly the ants would not have evolved herbivore defense behaviour unless their evolution had been affected by the plants.
Coevolution includes many forms of mutualism, host-parasite and predator-prey relationships between species as well as competition between species.