Lichens are actually two plants. There is a fungus, which provides most of the physical structure, and an algae which photosynthesises and makes food for both partners. A relationship like this where both partners gain is called a symbiosis.
Lichens are very sensitive to air pollution, especially sulphur dioxide. Many of the lichens disappeared around the big cities during the industrial revolution. Since the clean air acts of the 1950s & 60s the lichens have been returning. Here three types of lichen can be seen growing on hawthorn twigs by the stream.