In modern usage, hexapoda is used to refer to a class of small arthropods with a similar body plan: three body segments, with three pairs of legs on the central body segment. Up until the past few decades, in fact, "hexapoda" was used as a synonym for "insect". However, recently, in a move that was reminiscent of the demotion of Pluto to dwarf planet, the insects, which are marked by their possession of wings, were demoted to being a subclass of hexapoda, while the springtails, diplura and protura were put on an equal footing with insects.
There were taxonomic reasons for doing so, and various authorities (including the National Center for Biotechnology Information of the National Institute of Health) have deemed it to be so, but, much like Pluto's demotion, it doesn't change much. Most hexapoda that people will encounter will be obvious insects, such as the grasshopper and the butterfly, and the presence of the other three subclasses, all of which are tiny, hidden animals that can only be distinguished with a microscope, doesn't effect many people. But for technical discussions, "insects" are now a specific type of hexapoda.