Plug-in applications are programs that can easily be installed and used as part of your Web browser. Initially, the Netscape browser allowed you to download, install, and define supplementary programs that played sound or motion video or performed other functions. These were called helper applications. However, these applications run as a separate application and require that a second window be opened. A plug-in application is recognized automatically by the browser and its function is integrated into the main HTML file that is being presented.
Among popular plug-ins to download are Adobe's Acrobat, a document presentation and navigation program that lets you view documents just as they look in the print medium, RealNetworks' streaming video player, and Macromedia's Shockwave for DIrector, an interactive animation and sound player. There are now hundreds of possible plug-ins. Most users wait until they need a particular plug-in before they download it.
Also see plug and play.