Lichens are composite organisms and are symbiotic relationships between fungi (the mycobiont) and photosynthetic partners (photobiont) which are usually an algal form or a Cyanobacteria which are aquatic and photosynthetic and live in the water and can manufacture their own food. If an algae and a fungi meet and are compatible the fungal partner will create the main body of the lichen (known as a thallus) whilst the algal partner will begin to use sunlight to make sugars or food which will feed both the fungus and the alga. Lichens can occur in a variety of locations, including some of the harshest environments known, but don't tolerate pollutants which they accumulate from the air. The accumulation of these pollutants can interfere with the balance of the symbiotic relationship and eventually lead to a full breakdown of growth. Lichens grow very slowly and so care must be taken when taking specimens for identification.
When setting out identifying lichens is important to get to grips with the basic classification of growth forms, these being:-
Crustose (Paint-Like, somewhat flattish)
Gelatinous (producing a polysaccharide and absorbing and retaining water)
All lichens are considered to be edible (well at least not poisonous although a few could be so don't take the risk) and some are thought to be utter delicacies. Lichens are great contributors to the habitat with some making the nitrogen in the air which plants can use. Also as well as being important in the nutrient cycle in the places where they grow they can provide homes for mites and spiders and a variety of small insects.
People also use lichens for various medicines and they can produce a range of dyes.