You don't bring up a Web page and, when the script feels like it, it springs into action. Rather, scripts are triggered in response to some action. For example, you could write a script that runs when the user moves the mouse over an image on the page. Or you could write a script that runs when the user clicks on a button or selects an option from a drop-down list. The thing that triggers the script is called an event.
Many different events can occur when interacting with a Web page.
When the event occurs, the script that's designed to process the event runs. The script itself is called an event handler. Think of properties as adjectives that describe part of the Web page. For example, properties include the width and height of HTML elements, the URL of the document that's being displayed, and the number of frames that are on the page. Scripts can read and write properties.