<p>The plugins tool allows a module to allow <b>other</b> modules (and themes!) to provide plugins which provide some kind of functionality or some kind of task. For example, in Panels there are several types of plugins: Content types (which are like blocks), layouts (which are page layouts) and styles (which can be used to style a panel). Each plugin is represented by a .inc file, and the functionality they offer can differ wildly.</p>

<p>A module which uses plugins can implement a hook describing the plugin (which is not necessary, as defaults will be filled in) and then calls a ctools function which loads either all the known plugins (used for listing/choosing) or loads a specific plugin (used when its known which plugin is needed). From the perspective of the plugin system, a plugin is a packet of data, usually some printable info and a list of callbacks. It is up to the module implementing plugins to determine what that info means and what the callbacks do.</p>

<p>A module which implements plugins must first implement the <strong>hook_ctools_plugin_directory</strong> function, which simply tells the system which plugins are supported and what directory to look in. Each plugin will then be in a .inc file in the directory supplied. The .inc file will contain a specially named hook which returns the data necessary to implement the plugin.</p>