During the traditional animation process, key animators create the key frames -- the most important frames in the production. Clean-up animators then trace the key drawings onto new sheets of paper, removing any style idiosyncrasies and mistakes. Inbetweeners are less experienced animators who go through the film and draw whatever frames it needs between the key frames. This "tweening" process requires less skill than drawing key frames.
This type of production drawing has a loose, sketchy feel. Animators use it as a model for how the character looks and behaves. It can act as a first sketch for a later frame, establishing how the character moves and what expressions they display during a specific moment.
Another type of production drawing, the extreme drawing serves to show what characters look like during extremes of emotion or action. This is particularly important when using very cartoony characters, which have a great deal of "stretch and squash" and can change shape significantly when they move. Extreme drawings are usually a little more detailed than an animator's rough.
Collectors buy both types of production drawing for personal display. Not all drawings command the same price, however. Production art from more popular movies or studios usually brings a higher price than lesser-known art. Older production art also commands a higher price, especially "vintage" art dating from the 1970s or earlier. Most studios destroyed their production drawings before this time, and few examples survive. Collectors prefer cleaner, higher-quality artwork in which the character is distinctly recognizable, and drawing price rises accordingly for these pieces.