In the world of traditional (handdrawn) animation, the method most frequently used is called Pose To Pose Animation. First, the main Poses are drawn; the key poses (eg Arm up, arm down). After this, the animator draws the so called breakdowns; drawings needed to determine the arc of motion. eg arm in the middle; without this drawing there is no arc, but only a straight line between the upper- and lower arm key drawings. After this, the animator writes down the Charts; notations indicating the spacing of the drawings inbetween, better known as Inbetweens, to determine the acceleration and de-acceleration. These drawings are usually drawn by inbetweeners. If spaced out correctly, more drawings are used at the beginning and the end of the motion; the arm will start moving slowly, has the biggest velocity somewhere in the middle, and will then de-accelarate again to come to a halt. Inbetweens are mostly the mechanical part of the animation; inbetweeners are expected to stick exactly to the directions of the key-animator, who determines the exact timing of the scene. A Hold is also known as a Held Drawing. Animation (for film) runs at 24 Frames per second; if possible, animation is drawn on 'twos', meaning that each drawing is shot for two frames, resulting in twelve drawings per second (which still gives fluid motion), or, whan neccesary, on 'ones, where there is a seperate drawing for each and every fame of film. Usually, scenes consist of a mixture of ones and twos. Sometimes, in order to 'read' a pose or expression, the action needs to be frozen for a longer time; say eight frames or more. Often, it is better to have secondary characters in a scene not move too much, in order to not draw away the attention from the main character of the scene. As a rule, drawings that are exposed for a longer time, and as a result of that, do not move for a period of time, are called Holds. Sometimes longer holds are combined with an extra layer of animation, for instance a an extra layer with an animated cycle of hair waving in the wind; a method very frequently used in Japanese animation. Animation that uses a lot of holds - mostly TV-shows; cheap - is called limited animation; animation thas has fewer holds (and only for artistic reasons) is called full animation; for instance all of the Disney theatrical features. When strong, expressive expressions are called for, holds are also used in cartoon style 3D productions, such as Finding Nemo or Toy Story; I doubt if you will find any in more realistic 3D like eg Gollum in Lord Of The Rings. In the Walter Foster books series - available in Art Supply Stores - a Disney Animator who later worked for on several Tex Avery films called Preston Blair wrote two volumes on animation, describing all the main principles of animation; these books are good and do not cost much. More eleborate - and more expensive - are Disney Animation; The Illusion Of Life by Frank Thomas and Ollie Johnston, and The Animator`s Survival Kit by Richard Williams. I hope this answers your questions; it should more or less cover the 2D-animation part of the story.