Animations require some kind of timing mechanism that is responsible for updating the relevant property values at the right time. In WPF, this timing mechanism is catered for by the abstract Timeline class, which in short, represents a period of time. All of the available animation classes extend this class and add their own animation functionality.
When a Timeline class is used for animations, an internal copy is made and frozen, so that it is immutable. Additionally, a Clock object is created to preserve the runtime timing state of the Timeline object and is responsible for the actual timing of the animated property updates. The Timeline object itself does little other than define the relevant period of time.
The Clock object will be automatically created for us when we define a Storyboard object, or call one of the Animatable.BeginAnimation methods. Note that we do not typically need to concern ourselves with these Clock objects directly, but it can be helpful...