Plug-ins are programs that play or display files that your browser might encounter on the Web and would otherwise not know how to handle, such as sound or movie clips. Most popular plug-ins work with both Netscape Communicator and Internet Explorer, and some come preinstalled in your browser. If you're using Netscape Navigator or Communicator, you can see which plug-ins are already installed by selecting HelpûAbout Plug-ins.

Microsoft developed an alternative set of plug-ins based on Windows' ActiveX technology. But ActiveX applets (called controls) have yet to gain huge popularity on the Web, largely because they don't work smoothly with Communicator.

For plug-ins to work in Internet Explorer, the browser must be set to run both ActiveX controls and plug-ins: Select ViewûInternet Options and click the Security tab. Select Internet zone and click the Custom button, then the Settings button. Make sure that Run ActiveX controls and plugins is enabled.

Here's a guide to indispensable plug-ins. All are free and available on our cover CD as well as from the vendor. Typically they come as executable files, which you install as you would any other Windows package. Be forewarned that some files are very large (the Acrobat Reader, for example, is 5MB).

Adobe Acrobat Reader. This free viewer from Adobe Systems (www.adobe.com) lets you view and print documents stored in Adobe's Portable Document Format (.pdf), a spiffy desktop publishing format that's become standard on the Web.

Caption: Download the Adobe Acrobat Reader to view files in AdobeÆs portable document (.pdf) format

Cosmo Player. This VRML (Virtual Reality Markup Language) viewer from Silicon Graphics (www.cosmo.sgi.com) flies you through virtual spaces to play 3D games and view heavily animated Web sites. The Dilbert Zone (umweb1.unitedmedia.com/comics/dilbert/vrml) makes effective use of VRML (which the site calls Very Ridiculous Marketing Lingo).

QuickTime Viewer. Most movies on the Web are stored in Apple's QuickTime video format. In order to see them, you need Apple's QuickTime viewer (www.quicktime.apple.com).

RealPlayer. Enjoy video, sound, and animation with this multitalented multimedia player from RealNetworks (www.realaudio.com).

Shockwave Player. With Macromedia's Shockwave (www.macromedia.com), you can view product demonstrations (www.kodak.com:80/shocked.shtml) or participate in interactive games (slugfest.kaizen.net). Macromedia's page has links to thousands of "shocked" sites.