Coevolution is a special kind of evolution, wherein two or more species adapt to evolutionary changes in the other species. A common example includes long moth tongues and tube-shaped flowers, or hummingbird beaks and feeder flowers.
Coevolution is not necessarily a positive thing either. Parasites and pathogens will evolve in response to immune system changes in their host organisms. If the host develops a genetic immunity to a pathogen, it is likely that the pathogen will also evolve some kind of method to circumvent that immunity.