Tweening is a term used in the animation industry to describe the process of animating the transitional frames in between the key frames.
In a modern animation studio, work done in-house is usually restricted to key frames. Key frames are the important frames of animation found at the extremes of motion, that is, they're the frames from which all other motion can be implied. For example, a walking character's key frames would be the farthest extension of the legs in the stride. The important concept here is that the key frames represent only a small percentage of the total animation frames, but are the most important frames. There might only be a couple of key frames per second, while the animation might be made of 12 frames per second (note that this is half the frame rate of film).
Therefore, the workload on the artists drops considerably from around 6,000 frames of animation in an 8-minute short to less than 1,000 (depending on a number of factors, for example conversations require fewer key frames than lively action sequences).
Once all the key frames are assembled, the cells are shipped off to an animation studio for the much less skilled work of tweening. South Korea is famous for its tweening studios, where much of America's animation can be done with cheap foreign labor. The tweening artist looks at the key frames, decides what action goes in between them, and draws the intervening 6–11 frames based largely on copying the key frame. This is tedious, labor intensive, and very dull work. Ideally, the tweening artist doesn't include any flourishes or artistic expression whatsoever outside of what is provided by the key frames.
Because tweening is such an unskilled, tedious process, there have been attempts made to automate the process. Macromedia Flash, for example, has a built-in tweening feature to simplify the animation process. However, the feature can be tricky to use, resulting in bizarre transitions if key frames are not carefully selected. This is one of the inside jokes in the amateur Flash development parody "Powered by The Cheat" on Homestar Runner.
- Rumor, innuendo, and bits and pieces of information cobbled together from director's commentary tracks on cartoon DVDs. I don't actually have the slightest clue what I'm talking about and certainly no references or animation experience to speak of. However, I figure this is close enough to fool people who don't know any better.