A key difference from driver/extensions is that plugins generally add additional software functionality, whereas driver/extensions enable the use of some external hardware or software module.  The exceptions to this rule include plugins for PhotoShop which enable scanners to do their work, and Apple's own QuickTime, which can appear as a plugin for a browser such as Netscape's Communicator or Microsoft's Internet Explorer, as a self-contained media program extension, or as an independent application.  Plugins for browsers are often referred to as 'helper-applications'. You can see the overlap clearly.

To run many particular applications on your browser, you must acquire the relevant plugin, either from the software house such as RealNetworks or Macromedia or from the browser's company. It's simple to install them. Finding them is usually easy, but can occasionally be hard work.

Various plugins (Yahoo)
Adobe Photoshop plugins manufacturer list (Yahoo)