Tarantulas are a type of spider. They belong to the family Theraphosidae which includes 900 species of tarantula.
Spiders are divided into two main groups, the Araneomorphs or True Spider forms and the Mygalomorphs Tarantula form. The main difference is in the way the fangs are arranged. Araneomorphs have fangs that point diagonally and bite in a pinching motion. Mygalomorphs have fangs that point downward, and bite in a stabbing motion.
Araneomorphs include most spiders you find around, for example the orbweavers, jumping spiders, wolf spiders, grass spiders, and cobweb spiders are all araneomorphs.
Mygalomorphs include the tarantulas, trapdoor spiders, mouse spiders, purseweb spiders, and baldlegged spiders.