A lichen is a composite organism that emerges from algae or cyanobacteria (or both) living among filaments of a fungus in a symbiotic relationship. The whole combined life form has properties that are very different from properties of its component organisms.
Common names for lichens may contain the word "moss" and lichens may superficially look like and grow with mosses, but lichens are not related to mosses or any plant. They can survive in some of the most extreme environments on Earth: arctic tundra, hot dry deserts, rocky coasts, and toxic slag heaps. They can even live inside solid rock. Some lichens do not grow on anything, living out their lives blowing about the environment (see also: Aeroplankton).
Some lichens have lost the ability to reproduce sexually, yet continue to speciate. Lichens may be long-lived, with some considered to be among the oldest living things. Lichens can survive unprotected in space with no discernible damage.