Lycophytes comprise a group of plants which were formerly thought to be closely related to ferns, and grouped together with Horsetails (Equisetum) were often referred to as "Fern Allies". Lycophytes is a relatively new term coined to describe these plants. (Note: in this guide, Equisetum is included in the Ferns section).

Plants of the genus Selaginella are often referred to as Spikemosses in some countries, eg the USA, but it is not a term that has been in general use in Britain or Ireland. Equally, plants of the genus Huperzia are now often distinguished as Firmosses, but again this has yet to be widely adopted here. For now, I have grouped them together under the heading Clubmosses.

The division of clubmosses into different genera varies widely: some botanists use Lycopodium for all clubmosses other than Selaginella; otherwise there are anywhere from 3 to 12 genera depending on author. Here, I have followed the classification most often used in recent British guides.