Lichens are unusual creatures. A lichen is not a single organism the way most other living things are, but rather it is a combination of two organisms which live together intimately. Most of the lichen is composed of fungal filaments, but living among the filaments are algal cells, usually from a green alga or a cyanobacterium.
In many cases the fungus and the alga which together make the lichen may each be found living in nature without its partner, but many other lichens include a fungus which cannot survive on its own -- it has become dependent on its algal partner for survival. In all cases though, the appearance of the fungus in the lichen is quite different from its morphology as a separately growing individual.
For more about lichens, try the list of resources prepared by the American Bryological and Lichenological Society, or visit the World of Lichenology by Clifford Smith. If you're looking for a general introduction to lichen biology, you may also want to visit LichenLand at Oregon State University.
We also maintain a list of on-line Mycological and Lichenological Collection Catalogs which you can search for more information.
A collection of lichen images is available through the Virtual Foliage page at the University of Wisconsin. They have kindly allowed us to use a number of their images on these pages.