Declarative Animation and Storyboards
As you've seen, WPF animations are represented by a group of animation classes. You set the relevant information, such as the starting value, ending value, and duration, using a handful of properties. This obviously makes them a great fit for XAML. What's less clear is how you wire an animation up to a particular element and property and how you trigger it at the right time.
It turns out that two ingredients are at work in any declarative animation:
A storyboard. It's the XAML equivalent of the BeginAnimation() method. It allows you to direct an animation to the right element and property.
An event trigger. It responds to a property change or event (such as the Click event of a button) and controls the storyboard. ...