The class of animals called Arachnida, known as the arachnids, includes the spiders, mites, harvestmen, ticks, scorpions and pseudoscorpions. Most species of arachnid have eight legs attached to a cephalothorax, which comprises of the head and thorax fused together. What also sets them apart from other minibeasts such as insects are that morphologically they comprise of two body sections, the cephalothorax and the abdomen, and they do not have antennae or wings.
There are around 670 species of spider in Britain, 3,000 in Europe, and 48,000 that have so far been discovered globally. All of them play an important role in a balanced ecosystem. They help to keep the numbers down of other invertebrate pest species like flies, mites and lice by poisoning them by injecting venom and feeding on them. In turn are spiders are eaten themselves by birds or small mammals. Compared to mites and harvestmen, spiders are very familiar in that they will readily enter our homes, and are seen outdoors in elaborate webs of spun silk which they use to catch their prey. Other species do not spin webs, but will hunt their prey down.
For further reference see below:
British Arachnological Society (BAS)
Facebook British Spider Identification Group
Facebook UK Spiders Group
Britain’s Spiders A Field Guide by Bee, Oxford & Smith is an excellent reference book.
Collins Field Guide Spiders Britain and Northern Europe by Michael J. Roberts is also worth a look.