Behaviors in Microsoft Expression Web are scripting options that enable you to quickly add interactivity or increased functionality to text or other elements of your web page. You can customize built-in Expression Web Behaviors by using the Behaviors panel.
A Behavior is a combination of an event and an action that you want to occur when a site visitor interacts with a web page or an element of a page.
An event is a condition that causes a web browser to trigger a script. For example, moving the pointer over a graphic or clicking a link can trigger an event. Event names usually reflect how they are activated. For example, the onMouseOver event is triggered when the mouse pointer moves over a page element. Likewise, the onMouseClick event is activated when the user clicks a page element.
An action is an instruction that executes when an event occurs. Actions are functions — prewritten blocks of script code — that perform a specific task, such as swapping an image or opening a browser window. After you attach a script to a page element (by using the Behaviors panel), the browser "calls" the action that is triggered by the event.
Each action requires a set of properties and parameters in order to do a task. In Expression Web, you can view or edit the properties for each action using the Behaviors panel.