These are the extremes animated by Doug Compton.
This is a good sequence to look at to understand how drawn animation works. It's very simple and straight ahead. Since there's no character acting, motion here is broken down to its essentials. Even so there's "character" to the movement.
The scene eventually will be on "twos" -one drawing every two frames. To start, however, Doug has drawn approximately one drawing for every 10 frames.
Those blanks are filled in by an inbetweener.
In this sequence, you don't even need particularly strong drawing skill to create inbetweens. I encourage everyone to print out a few (just click to enlarge) and try it.
Even though animation is a difficult process, if everyone does their part as well as Doug did his here it's not all that hard.