Plugins provide a way for developers to modify or extend the behavior of applications within the IPS Community Suite.
This guide will cover in detail the different features available within Plugins. Many of the features describes are accessed from the Plugin Developer Center - to access this, first place IPS Community Suite in developer mode. Once developer mode is enabled, a "Create Plugin" button will appear in the Admin CP under System --> Site Features --> Plugin. Use this tool to create your plugin, after which you'll be taken into the Plugin Developer Center.
The purpose of plugins
Plugins differ from applications in that they are designed to extend existing applications, and the features available for use by plugins reflect that. There are code hooks and theme hooks, which allow you to piggyback off of code that exists in other applications and manipulate the output - great for adding new features. If you find yourself adding entirely new functionality that isn't a subset of something an existing application does, you may want to consider building an application of your own instead.
While in developer mode, plugins are created as files on the filesystem, and this guide will cover the available features and how to use them. When your plugin is ready for distribution, IPS4 will compile it as a single XML file that can be shared and installed by others, for example in the IPS Marketplace. Simply download the XML file from the Plugin Developer Center.