Hexapods, subphylum Hexapoda, is a group of arthropods that includes the insects and some close relatives, including springtails, proturans, and diplurans. Hexapods are probably monophyletic, meaning they all descend from a common ancestor, and include no members that didn't descend from that ancestor. However, there is some evidence that springtails may have evolved their features independently, in an example of convergent evolution, although this is not the majority position. The relationships between members of Hexapoda is not entirely known — there are three primary cladistic hypotheses. Most other arthropods, such as spiders and crustaceans, have more than six legs.
Hexapods' signature characteristic is three pairs of legs, which is where they get their name (hexa means "six"), but they share many other features in common, including a consolidated thorax and genetic similarity. Hexapods diverged from their sister group, the fairy shrimps, around 440 million years ago, during the Silurian period, shortly after the appearance of terrestrial vascular plants. Little is known about how hexapods diverged from fairy shrimp and began to walk on land. Much more is known about the early history of the tetrapods. Prior to the arrival of vascular plants and hexapods on land, it was largely barren.
Hexapods are found on every continent on Earth, including Antarctica. In particular, springtails have been discovered in some of the most hostile environments on Earth, such as the McMurdo Dry Valleys in Antarctica, which contain so little moisture and life that large animals that die there dry out and are mummified instead of decomposing, like pretty much everywhere else.
The most familiar hexapods to humans would be the insects: flies, bees, mosquitos, beetles, mites, etc. In terms of species, hexapods are the most diverse subphylum on the planet, with over a million described species, and estimates of total species between 5 and 8 million. In particular, there are over a million species of beetle, with new species being discovered all the time. In all, beetles consist of about 40% of insects, and roughly 25% of all known life forms. If you randomly selected a species of animal from the Earth's surface, it's quite likely it would be a beetle.