"Inbetweening" - Traditional vs Digital

Traditional

Traditional inbetweening involves the use of light tables to draw a set of pencil-on-paper pictures.
In the inbetweening workflow of traditional hand-drawn animation the senior or key artist would draw the keyframes which define the movement, then, after testing and approval of the rough animation, would hand over the scene to his or her assistant. The assistant does the clean-up and the necessary inbetweens, or, in large studios, only some breakdowns which define the movement in more detail, then handing down the scene to his assistant, the inbetweener who does the rest.

Digital

When animating in a digital context, especially with Adobe Flash, the shortened term tweening is commonly used. Sophisticated animation software enables one to identify specific objects in an image and define how they should move and change during the tweening process. Software may be used to manually render or adjust transitional frames by hand or may be used to automatically render transitional frames using interpolation of graphic parameters. In the context of Adobe Flash, inbetweening using automatic interpolation is called tweening, and the resulting sequence of frames is called a tween.
"Ease-in" and "ease-out" in digital animation typically refer to a mechanism for defining the 'physics' of the transition between two animation states, eg the linearity of a tween.
The use of computers for inbetweening was pioneered by Nestor Burtnyk and Marceli Wein at the National Research Council of Canada. They received a Technical Achievement Academy Award in 1997, for "pioneering work in the development of software techniques for computer assisted key framing for character animation".

Bron: wikipedia, http://en.wikipedia.org/wiki/Inbetweening