Pose to Pose and Straight Ahead are the two main ways of animating a sequence and most animators will use a mix of the two of these in their pieces.
Straight Ahead Animation
This is when every frame of the sequence is drawn as you go. This is best for things which are unpredictable such as water or fire. It also gives animators more freedom to explore and discover as their work progresses.
Pose to Pose Animation
This is when the main points of the animation, the beginning, middle and end are drawn in first, then the animator goes back and draws the poses in between these, this gives the animator more control over the whole piece.