A plugin (or plug-in) is a computer program that can interact with another program to provide a certain, usually very specific, function. Typical examples are plugins to display specific graphic formats (e.g., SVG if the browser doesn´t include this format by default), to play multimedia files, to encrypt/decrypt email (e.g., PGP), or to filter images in graphic programs. The main program (a web browser or an email program, for example) sets a standard for the data exchange with a plugin, makes data available to the plugin and acts on the results of the plugin's processing.
A pluginbrery is a plugin library used for all the web browsers in an operating system. So, instead of installing similar plugins for different browsers in the same operating system, the user can install the same plugin (in the pluginbrery) for all the Internet browsers. This can be used for functionality including: