Straight ahead animation starts from the first drawing, and works to the end scene through drawing to drawing. Things such as size, volume and proportions may be inconsistent however it allows spontaneity to scenes (fast action scenes are normally done this way).
Pose to Pose is more planned, key drawings are shown in intervals throughout the scene. This type gives better control of volume, size and proportions and so is the actual action itself. Normally a key animator works on the key poses and hands it to an assistant who fills in the between frames.