Lichens are 'dual organisms'. Every lichen is a partnership between members of two different kingdoms which live together in a special, mutually beneficial relationship - a symbiosis. Each lichen is made up of a fungus and an algae.
The body of the lichen is built up by tough fungal hyphae, and the algae live inside that framework.
The fungus protects the algae from the harsh world outside, and provides it with water and mineral nutrients. The algae makes its own food by photosynthesis, and leaks some of this food, which is then absorbed by the fungus, which cannot make its own food.
The partnership is so tough and self-reliant that lichens can grow in places like bare rock in deserts, where nothing else can survive. When it is too dry, too hot, or too cold, lichens go into a state of suspended animation until conditions improve.
Since the algae make up only about 5% of each lichen, and are out of action for much of the time, you can imagine that lichens grow very slowly - only a few millimetres per year. They make up for this by living for centuries, or in a few cases, millennia.
Lichens have only one serious weakness - they must absorb their mineral nutrients from the rain. So if the air is polluted with sulphur dioxide, this dissolves in the rain and is absorbed by the lichens which often die as a result.