A group of organisms that are symbiotic associations (see symbiosis) between a fungus (usually one of the Ascomycota) and a green alga or a cyanobacterium. The fungal partner (mycobiont) usually makes up most of the lichen body and the cells of the alga or bacterium ( phycobiont) are distributed within it. The phycobiont photosynthesizes and passes most of its food to the fungus and the fungus protects its partner's cells. The lichen reproduces by means of soredia, isidia, or by fungal spores, which must find a suitable partner on germination. Lichens are slow growing but can live in regions that are too cold or exposed for other plants. They may form a flattened crust or be erect and branching. Many grow as epiphytes, especially on tree trunks. Some species are very sensitive to air pollution and have been used as indicator species. Lichens are classified as fungi, usually being placed in the taxon of the fungal partner; some authorities group them together in the phylum Mycophycophyta.
http://www.nhm.ac.uk/nature-online/environmental-change/lichens/lichens-env.html Introduction to lichens compiled by the Natural History Museum