Insects (Class Insecta), from Latin insectum (animal) "with a notched or divided body," make up the largest class of arthropods. The study of insects is called entomology.

Insects are arthropods, having a hard exoskeleton, a three-part body (head, thorax, and abdomen), three pairs of jointed legs, compound eyes, and two antennae. The head holds large compound eyes, antenna (feelers), and the mouthpart. The legs (and wings, if applicable) are attached to the thorax.

Insects are the most diverse group of animals on the planet. There are approximately 2,200 species of praying mantis, 5,000 dragonfly, 20,000 grasshopper, 82,000 true bug, 120,000 fly, 110,000 bee, wasp, ant and sawfly, 170,000 butterfly and moth, and 360,000 beetle species described to date. Estimates of the total number of current species, including those not yet known to science, range from two million to fifty million, with newer studies favouring a lower figure of about six to ten million. With over a million described species—more than half of all known living organisms—insects potentially represent over 90% of the differing life forms on the planet. They are most diverse at the equator and their diversity declines toward the poles. Insects may be found in nearly all environments on the planet, although only a small number of species occur in the oceans, a habitat dominated by another arthropod group, the crustaceans.