This common name refers to only one class.
These are pretty much always called Worms.
The subclass Oligochaeta in the class Clitelatta contains worms.
Clitelatta-Oligochaeta (Worms) Worm Adult
View 2 PicturesI know most people know what a worm looks like, but when trying to tie an imitation, the memory is often a poor source of accurate color information, and a picture helps to get the right shade. One thing that surprised me in my sampling is that quite a few worms did turn up. Before I was a fly fisherman, when I used worms for bait, I always thought they were merely a tantalizing morsel and not a regular trout food, but I've since realized that there are quite a few worms that live in the sediment on a stream bottom.