Tweening

An animation term.

Animation is nothing more than a series of frames that have slightly different images from frame to frame. Flicking through the frames rapidly gives the impression of a moving image.
The problem is having to create the slightly changing images on every frame in the first place. The hard way would be to hand draw every one of them.

When the film 'Snow White' was created way back in 1937 (considered one of the best 100 American films of all time) every single frame was carefully hand drawn and coloured - a task that took 3 years of meticulous effort.

An easier way is to use the 'tweening' feature of an animation application.

For example Flash has a tweening feature. It begins by you providing a starting frame, called a 'key frame' with a vector image in place. Then you define the last frame in the sequence (also called a key frame) with the final vector image in place, the tweening feature then enables the computer to automatically draw the required images in every frame in-between to give the impression of a smooth animation.