WordPress Plugins allow easy modification, customization, and enhancement of a WordPress blog. Instead of changing the core programming of WordPress, you can add functionality with WordPress Plugins. Here is a basic definition:
WordPress Plugin: A WordPress Plugin is a program, or a set of one or more functions, written in the PHP scripting language, that adds a specific set of features or services to the WordPress weblog, which can be seamlessly integrated with the weblog using access points and methods provided by the WordPress Plugin Application Program Interface (API).
One of the mothership pages to get you started.
Check out Hookr.io for the most comprehensive collection of WordPress hooks, actions, filters, classes, constants, functions and shortcodes. You’ll be spending much of your free time here. They are now tracking plugins as well.
The Codex is the heart and soul of WordPress.
“The WordPress CMS has changed the face of our Internet and allowed a surge of new ideas to prosper and its open source movement holds a strong presence rooted in software and web development. WordPress is a blogging platform which has the ability to launch into many other scripts such as web forums, job boards, and even a classic webpage Content Management System.”