Plugin

Plugins are ways to extend and add to the functionality that already exists in WordPress.

The core of WordPress is designed to be lean and lightweight, to maximize flexibility and minimize code bloat. Plugins then offer custom functions and features so that each user can tailor their site to their specific needs.

For instructions and information on downloading, installing, upgrading, troubleshooting, and managing your WordPress Plugins, see Managing Plugins. If you want to develop your own plugin, there is a comprehensive list of resources in Plugin Resources.

From: http://codex.wordpress.org/Plugins