An extension is a piece of software that can be added to a Macromedia application to enhance the application's capabilities. Macromedia offers several types of extensions; for example, Dreamweaver and UltraDev extensions include everything from HTML code that can be added to the Object palette and the Insert menu, to JavaScript commands that you can add to the Command menu, to new behaviors, Property inspectors and floating palettes. Flash 5 extensions include Smart Clips that you can use to build projects, reusable ActionScript code snippets, additional symbol libraries, and even customized Generator objects. All Macromedia applications that support the Macromedia Extension Manager can use font extension packages that install a font to your hard disk.

The Macromedia Exchange Web site is a repository for many kinds of extensions. Some extensions on the Macromedia Exchange Web site are created by Macromedia and some are created by other developers. If you find an extension you're interested in on that site, you can download it using your Web browser and then install it in an application using the Macromedia Extension Manager.