Tarantulas comprise a group of hairy and often very large arachnids belonging mainly to the family Theraphosidae, of which approximately 900 species have been identified. Historically tarantulas were the bigger genera from the family Lycosidae (like Lycosa tarantula). The colonists of the Americas gave the name to the bigger spiders of the tropic-dwelling families Theraphosidae and Dipluridae (funnel-web tarantulas), and that usage has now supplanted the earlier European one.

Some genera of tarantula hunt prey primarily in trees; others hunt on or near the ground. All tarantulas can produce silk—while arboreal species will typically reside in a silken "tube tent", terrestrial species will line their burrows with silk to stabilize the burrow wall and facilitate climbing up and down. Tarantulas mainly eat insects and other arthropods, using ambush as their primary method of prey capture. The biggest tarantulas can kill animals as large as lizards, mice, and birds. Tarantulas are found in tropical and desert regions around the world. Most tarantulas are harmless to humans, and some species are popular in the exotic pet trade. All tarantulas are venomous, but only some species (while not known to have ever produced human fatalities) have venom that can produce extreme discomfort over a period of several days.

The name 'tarantula' comes from the town of Taranto in Southern Italy and was originally used for an unrelated species of European wolf spider. The name was borrowed to apply to the theraphosids when Europeans explored areas where these large spiders were common. In Africa, theraphosids are frequently referred to as "baboon spiders". Asian forms are known as "earth tigers" or "bird-eating spiders". Australians refer to their species as "barking spiders", "whistling spiders", or "bird-eating spiders". People in French-speaking areas may apply the general name "mygales" to theraphosid spiders.

Like all arthropods, the tarantula is an invertebrate that relies on an exoskeleton for muscular support. A tarantula’s body consists of two main parts, the prosoma (cephalothorax) and the opisthosoma (abdomen). The prosoma and opisthosoma are connected by the pedicle, or what is often called the pregenital somite. This waist-like connecting piece is actually part of the prosoma and allows the opisthosoma to move in a wide range of motion relative to the prosoma.

Depending on the species, the body length of tarantulas ranges from 2.5 to 10 centimetres (1 to 4 in), with 8–30-centimetre (3–12 in) leg spans. Leg span is determined by measuring from the tip of the back leg to the tip of the front leg on the opposite side. The largest species of tarantula can weigh over 85 grams (3 oz). The largest of all, the Goliath Birdeater (Theraphosa blondi) from Venezuela and Brazil, has been reported to have a weight of 150 grams (5.3 oz) and a leg-span of up to 30 centimetres (12 in), males being the longer and females greater in girth.

T. apophysis (the Pinkfoot Goliath) was described 187 years after the Goliath Birdeater; therefore its characteristics are not as well attested.T. blondi is generally thought to be the heaviest tarantula, and T. apophysis to have the greatest leg span. Two other species, Lasiodora parahybana (the Brazilian Salmon Birdeater) and L. klugi, rival the size of the two Goliath spiders.

The majority of North American tarantulas are brown. Elsewhere have been found species colored cobalt blue (Haplopelma lividum), black with white stripes (Aphonopelma seemanni), yellow leg markings (Eupalaestrus campestratus), metallic blue legs with vibrant orange abdomen and greenbottle blue (Chromatopelma cyaneopubescens). Their natural habitats include savanna, grasslands such as the pampas, rainforests, deserts, scrubland, mountains, and cloud forests. They are generally divided into terrestrial types that frequently make burrows and arboreal types that build tented shelters well off the ground.

Tarantulas are becoming increasingly popular as pets and are readily available in captivity. Most tarantulas are not strongly venomous to humans but their bite can be fatal to those with allergic reactions.