Spiders, numbering some 36,000 known species, belong to a huge group of invertebrates called arthropods. So do insects, crustaceans, centipedes, millipedes, and other animals characterized by paired and jointed legs (which is what the word "arthropod" means), segmented body, and an exoskeleton.
A lot of people think spiders are insects, but the two are only distantly related. Spiders share a closer kinship with scorpions, ticks, mites, daddy longlegs, and other arthropods that have, as their most obvious characteristic, eight legs arranged in four pairs. These eight-legged arthropods are called arachnids. Insects, in contrast, have six legs arranged in three pairs.
Besides having eight legs, spiders and other arachnids have an extra pair of appendages called pedipalps. Pedipalps are a little like hands: they help arachnids feel their surroundings and hold on to prey and other objects.