Ferns and related plants (clubmosses, quillworts and horsetails) are an evolutionary ancient group of plants. Fossils of such plants have been found dating back 400 million years. Terrestrial plant life of the Carboniferous period (360 to 300 million years ago) was dominated by Pteridophyta (ferns and their allies). Clubmosses and horsetails are generally small plants today, but in the Carboniferous period some species towered tens of metres high. Today there are around 12,000 species of Pteridophytes, found in a huge range of habitats from semi-deserts and other arid regions to sea-sprayed cliffs and freshwater ecosystems. However, the vast majority of species are found in both temperate and tropical rainforests.

Pteridophyte reproduction remained a mystery for centuries. They lack flowers nor do they produce seeds like the vast majority of plants (angiosperms) found worldwide during the timescale of human existence. It is thought that this led ancient cultures to believe they were magical plants; Druids thought they had tiny invisible flowers produced only on midsummer's eve. It is now known that pteridophytes produce spores, tiny near-microscopic cells which are the fern equivalent of seeds. Spores are produced in sporangia found on the underside of the leaves, known as fronds. Some pteridophytes form large colonies from creeping stems. For example, a whole hillside covered in bracken may in fact be one single organism.

Bracken is the most well known and highly abundant fern found in Britain, common particularly on moorland. Bracken is in fact a genus (Pteridium) of around 10 species of coarse ferns. They are probably the most widely distributed genus of ferns found on all continents except Antarctica. They are an interesting group of plants in that they exhibit alleopathy, producing chemicals that inhibit the growth of other plant species. This allows them to dominate other vegetation, particularly during re-growth after a fire. Tree growth may be inhibited for some time after bracken is removed from an area, as the toxins can remain in the soil. However, bracken can be beneficial to some British plant species. They provide a substitute shady habitat for bluebells and wood anemones where woodland no longer exists.

There are over 100 species of ferns in Britain alone, and with a little practice many can be identified with little botanical knowledge. A good starting point is the FSC AIDGAP guide to British ferns designed to easily identify species with no scientific botanical knowledge. For a more detailed text, the Ferns of Britain and Ireland (Cambridge University Press) is a popular comprehensive field guide. Not all species can be identified by features visible to the naked eye alone. A decent hand lens or even dissecting microscope can be hugely important in viewing key identifying features such as sporangia and leaflet shapes.