Though software plug-ins might not make your room smell as nice as the scented ones you stick in an outlet, they are still useful. A software plug-in is an add-on for a program that adds functionality to it. For example, a Photoshop plug-in (such as Eye Candy) may add extra filters that you can use to manipulate images. A browser plug-in (such as Macromedia Flash or Apple QuickTime) allows you to play certain multimedia files within your Web browser. VST plug-ins add effects for audio recording and sequencing programs such as Cubase and Logic Audio.
Most graphics and audio programs today support plug-ins since they are a convenient way to expand the capabilities of the program. Though some plug-ins may be shipped with the program, most are developed by third-parties and are sold separately. Because companies that make browser plug-ins are often competing for a standard (such as Flash and QuickTime), these plug-ins are usually available as free downloads from the Internet.
A list of plug-ins and their associated file extensions can be found here: Plugin File Formats.