Lichens are symbiotic creatures that are comprised of both fungi and algae. Typically, a lichen is comprised of a fungal filament with green algae living inside of the filament. Cyanobacteria may also reside inside of the fungal filament to complete the lichen.
In many cases, these two separate organisms that make up a lichen can live alone, but often, they choose to live together symbiotically in order to protect each other and draw nutrients from each other. Sometimes, however, the fungal component cannot survive independently. Lichens come in many varieties, with a lichen's appearance being dependent on the composition of both of its unique organisms.