WordPress has a rich plugin architecture which allows users and developers to add to or extend its functionality beyond the features that come as part of the base install.
Plugins are php programs that add or extend functionality via WordPress’ system of hooks and filters. They can range from adding a shortcode or fetching a RSS feed, to large blocks of code that duplicate functionality of commercial programs.
Plugins typically use the GPL2 license. Although this is not required for self-hosted plugins, plugins that are hosted in the Wordpress repository must be GPL.
The wordpress-plugin tag is for specific programming questions related to plugin development. Questions are also typically tagged php or with the API they are trying to consume (if applicable).