A plug-in is a piece of software that adds tools and features to already-existing software. Typical examples of plug-ins are those that enhance or enable audio, video and visual effects. Plug-ins are sometimes called add-ons, which is the term that the Mozilla Firefox 
browser uses.
Plug-ins are available for a variety of software. Examples of pieces of software that have plug-ins readily available online are Windows Media Player, WinAmp , Quicktime and even Adobe Photoshop. Most of the time, plug-ins are associated with browsers, for example, a plug-in for blocking online advertisements.
The FlashPlayer and DivX Player (Digital Video Express) plug-ins for browsers allow one to view videos embedded within a Web page. Mozilla Firefox has a variety of plug-ins that have various uses ranging from adding weather reports to your status bar to synchronizing your bookmarks.
Another popular plug-in is PGP (Pretty Good Privacy) for e-mail clients like Thunderbird or Microsoft Outlook. This plug-in allows the sender to digitally sign a message and even encrypt the message, so that only the intended recipient can decrypt it.
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