When organisms evolve together based on mutualistic relationships. The organisms both benefit from their interaction with one another. They may rely on each other for things like nutrients or protection.Edit
"Natural selection cannot possibly produce any modification in a species exclusively for the good of another species" (Darwin, On the Origin of Species)- Here Darwin argues that altruism between species doesn't exist, that instead, natural selection will produce modifications that can benefit another species while also benefitting the original species.
Symbiotic mutualism: When two different species are involved with one another for the bulk of their life span (Futuyma, 2005). Edit
Mycorrhizal fungi and plants- while this relationship can be considered mutualistic when soil is infertile, it can be parasitic and reduce plant growth when soil is fertile (Futuyma, 2005).
Clown fish & Sea anemone (http://bio4esobil2011.wordpress.com/2012/05/27/mutualism/)
Bee & Flower http://bio4esobil2011.wordpress.com/2012/05/2
Darwin, C. (2003). On the Origin of Species: A Facsimile of the First Edition. Cambridge, MA: Harvard University Press.
Futuyma, D. (2005). Evolution. Sunderland, MA: Sinauer Associates, Inc.