Spiders are predatory invertebrate animals that produce silk, and have two tagma, eight legs, no chewing mouth parts and no wings. Spiders are classified in the order Araneae, one of several orders within the larger class of arachnids, a group which also contains scorpions, whip scorpions, mites, ticks, and Opiliones (harvestmen). The study of spiders is known as arachnology, and those who study spiders are arachnologists.

Many spiders hunt by building webs to trap insects. These webs are made of spider silk, a thin, strong protein strand extruded by the spider from spinnerets most commonly found on the end of the abdomen. For its weight, spider silk is five times stronger than steel. All spiders produce silk, although not all use it to spin elaborate traps. Silk can be used to aid in climbing, forming smooth walls for burrows, building egg sacs, wrapping prey, temporarily holding sperm and for many other applications.

All but one family of spiders (Uloboridae) have the ability to inject venom in order to kill and liquefy prey and also to protect themselves. Only a limited subset of spiders can produce significant medical problems by biting humans. Many other of the larger kinds of spiders can give bites that cause discomfort that may continue for some time but will not produce lasting effects.