What are these annoying things that you need to play the theme music of a newly discovered X-Files page or watch the video clips of stars on E!Online or see the animations in this fantastic tutorial? Plug-ins are the key to the cool stuff.

A plug-in application is a small piece of software that enhances the capabilities of the larger piece of software into which it is "plugged". You can usually download plug-ins for free from the Web and store them locally. Examples of common plug-ins for Web browsers are Real Audio's streaming sound player, Macromedia's Shockwave for Director and Adobe Acrobat.