Fungi have no chlorophyll to convert the sun's energy into
food; they have to rely on other plant and animal material, and they
use enzymes to dissolve their food. In many ways, fungi are more
closely related to animals than to plants.
Toadstools and mushrooms (there is no clear distinction between these two terms) may rot away after a few days, but they are just the fruiting bodies connected to long-lived underground fungal threads, called hyphae. The hyphae combine to form a mycelium, sometimes in the form of an expanding disc. Some types of mycelia can live for hundreds or even thousands of years.
A 'fairy ring' consists of fruiting bodies emerging around the edge of a mycelial disc that expands from its point of origin. The diameter of the ring gives a rough guide to the age of the fungal organism (the mycelium - not the mushrooms, which are merely its fruitbodies).
Of the 4000 or more large fungus species found in the British Isles,
only a small minority are poisonous, but they can be deadly. That is
quite sufficient to justify the ultimate in caution. If in any doubt about
the identity of a mushroom or other fungus, play safe and do not
eat any of it without first obtaining expert advice and a definite
identication.
Gathering a meal from the wild is one of the pleasures of the countryside. Unfortunately several edible fungi species can cause tummy upsets for some people even though others enjoy them with no evident undesirable effects. Some (morels, for example) are edible only when cooked: cooking breaks down the toxins within these fungi. If you are new to collecting edible fungi, whenever in doubt consult an expert or leave well alone.