Plugin

A plugin (or "plug-in") is a computer program that interacts with another program to provide a certain function. Typical examples of plugins include: a web browser plugin (such as "adobe acrobat reader"), a program to display specific graphic formats (such as "video codecs" for Windows Media Player), and the like.

Plugins are slightly different from program extensions, which modify or add to existing functionality. The main difference is that plugins generally do not change a user interface and run within well-defined boundary; comparatively, extensions generally have less restrictions on their actions and may provide their own user interfaces.

Perhaps the first applications to include a plugin function were HyperCard and QuarkXPress on the Macintosh, both released in 1987. These days, plugins are typically implemented as shared libraries that need to be installed in a place where the application will find them. HyperCard supported a similar facility, but it was more common for the plugin code to be included in the HyperCard documents (called stacks) themselves. This way, the HyperCard stack became a self-contained application in its own right, which could be distributed as a single entity that could be run by the user without the need for additional installation steps.

This article is adapted from: wikiPedia.com