There were no copier machines, and certainly no computer graphics, when cell animation first began, so every cell was hand-painted by an artist. When artists uses the see-thru material, they could trace the previous cell and then make the slight adjustment for movement required for the next cell.
One element of early cell animation that was fixed was the background. An artist could draw a forest, a pasture or a city jail background and spend a lot of time getting each detail right because it would be used behind the photograph of each animation cell. In other words, the artist might hand draw 50 drawings of a mouse to make him walk across the screen, but he would need only one background to put behind each cell as it was shot.
Early on, animators realized that they could get one perfect segment of a character walking or skipping and then use it over and over in that short cartoon and in others using the same character. For instance, if a character such as Mickey Mouse would whistle while he walked, that filmed animation image, once individually drawn and filmed, could then be used over again in any sequence where Mickey would walk and whistle.
Before using cells, animation used individual drawings on regular paper. Backgrounds had to be added later. Fine detail was eliminated. But with the use of cells with their see-thru quality, an animator could get more realistic details because he could see the work on the previous cell and then make tiny changes on the next. This is where Disney began to do things like Snow White's hair swinging or the feathers of a birds moving.
Collecting these cell drawings, whether by Disney or another studio, can be a fun and moderately priced hobby. The drawings make great wall art when framed and some can be found that have been colored for the later color photography. If you are lucky enough to find an early Disney animation cell, you truly have a piece of film and American history.