Inbetweening is the process of generating intermediate frames between two images to give the appearance that the first image evolves smoothly into the second image. Inbetweens are the drawings between the key frames which help to create the illusion of motion. Inbetweening is a key process in all types of animation.
Typically, an animator does not draw in-betweens for all 24 frames required for one second of film. Only very fast movements require animation 'on ones', as it is called. Most movements can be done with 12 drawing per second, which is called animating 'on twos'. Too few in-betweens distort the illusion of movement, such as in cheap TV animation series where there can be as few as 4 drawings for a second of film.
Motion Blur is the apparent streaking of rapidly moving objects in a still image or sequence of images such as a movie or animation. It results when the image being recorded changes during the recording of a single frame, either due to rapid movement or long exposure.
Onion Skinning is a 2D computer graphics term for a technique used in creating animated cartoons and editing movies to see several frames at once. This way, the animator or editor can make decisions on how to create or change an image based on the previous image in the sequence.
In traditional cartoon animation, the individual frames of a movie were initially drawn on thin onionskin paper over a light source. The animators (mostly inbetweeners) would put previous and next drawings exactly beneath the working drawing, so that they could draw the 'in-between' to give a smooth motion.
Tutorial on Tweening in ActionScript 3 http://www.flashcomponents.net/category_tutorials/page/1.html
A set of graphs showing 30 different easing curves http://hosted.zeh.com.br/tweener/docs/en-us/misc/transitions.html
(Images both source from wikipedia.com)
