Tarantula are a group of hairy and often very large spiders belonging to the family Theraphosidae, of which approximately 900 species have been identified. Some genera of the tarantulas hunt prey primarily in trees and other genera 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. 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 other parts of the world also 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 or the cephalothorax (the former is most often used) and the abdomen or opisthosoma. 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 2.5–10 cm (1–4 inches), with 8–30 cm (3 to 12 inch) leg spans (their size when including their legs). Leg span is determined by measuring from the tip of the back leg to the tip of the front leg on the same side, although some people measure from the tip of the first leg to the tip of the fourth leg on the other side. The largest species of tarantula can weigh over 9.1 grams (0.3 ounces). One candidate for the title of the largest of all species, the Theraphosa blondi (goliath birdeater) from Venezuela and Brazil, has been reported to have a weight of 3 ounces and a leg span of up to 11 inches (28 cm). The males have the longer length, and the females have broader girth.

Theraphosa apophysis (the pinkfoot goliath) was described 187 years after the Goliath birdeater; therefore its characteristics are not as well attested. However, leg spans of up to 33 cm (13 inches) have been reported for that species. 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 Lasiodora klugi, get very large and rival the size of the two Theraphosa species.

The majority of North American tarantulas are brown. Many species from other parts of the world have more extensive coloration patterns, ranging from cobalt blue (Haplopelma lividum), black with white stripes (Eupalaestrus campestratus or Aphonopelma seemanni), to metallic blue legs with vibrant orange abdomen (Chromatopelma cyaneopubescens, green bottle blue). 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.