This common name refers to only one class.
These are pretty much always called Springtails.
Collembola are actually a subclass of a class called Entognatha. They are the most common of the only six-legged arthropods (Hexapods) that aren't insects. Many Collembolans are terrestrial (Terrestrial: Insects which live on land and are fed on by trout only when they incidentally fall into the water are known as "terrestrials" to fly anglers, and they're very important in late summer.), but some live on the surface of their water, and they can be a supplemental food source for trout, especially young-of-the-year trout.
They rarely grow longer than 6mm or occur in trout streams in such numbers that an imitation with a fly is warranted, but they are interesting creatures.