Straight Ahead Action starts at the first drawing and works drawing by drawing to the end of a scene. Size, volume, and proportions can drift with this method, but it does have spontaneity and freshness. Fast, wild action scenes are done this way.
Pose to Pose is more planned out and charted with KEY drawings done at intervals throughout the scene. The lead animator can then turn charting and keys over to an assistant. The use of KEY FRAMES or EXTREMES is the computer equivalent and can be found in software including Flash, After Effects and even Photoshop’s animation palette. The application can TWEEN certain parameters (create the inbetweens that the traditional assistant used to draw) to fill in the gaps. To further stress the carefully drawn KEY poses, many of the inbetween drawings tend to be very similar to the KEY drawings created by the lead animator with just a few rather sketchier linking drawings (some Disney animators even experimented with blurs). See Slow In and Slow Out
Often scenes may employ both methods of animation.