In the next part of his WP7 series, Jesse Liberty discusses WP7 animations.
Source: Jesse Liberty's Blog
Animation in Silverlight is declared in Storyboards and triggered by a user’s action, by a background action or process or, most commonly, by a change in state. Thus an animation might be triggered by pressing a button, but more often the animation will ease the user’s perception of the transition from one state such as the mouse-over, to another, such as mouse press.