As you've seen, motion can happen for a variety of reasons, but the basics of every motion are the same. All objects move because they are affected by forces, and all moving objects have momentum that makes it difficult to change their course. Additionally, the language of motion defines a number of other specific motions that help bring characters to life, such as anticipation, overshoot, and follow-through.
The best way to understand these concepts is to practice. Animate a lot of very simple scenes, using the fundamental motions described in this chapter.
Chapter One. Basics of Character Design
Chapter Two. Modeling Characters
Chapter Three. Rigging Characters
Chapter Four. Basics of Animation
Chapter Five. Creating Strong Poses
Chapter Six. Walking and Locomotion
Chapter Seven. Facial and Dialogue Animation
Chapter Eight. Animal Motion
Chapter Nine. Acting
Chapter Ten. Directing and Filmmaking