WordPress is a free and open-source website content management system (CMS). Features include a website design templates and plugins and is most frequently associated with blogging. Used by more than 60 million websites, including 30.6% of the top 10 million websites making WordPress the most popular website management system.
WordPress users may install and switch among different themes which allow users to change the look and functionality of a WordPress website without altering the core code or site content. Every WordPress website requires at least one theme to be present and every theme should be designed using WordPress standards with structured PHP, valid HTML (HyperText Markup Language), and Cascading Style Sheets (CSS).
Themes are installed using the WordPress "Appearance" administration tool in the dashboard or by way of folders being uploaded directly into the themes directory, typically byFile Transfer Protocol. The PHP, HTML and CSS found in themes can be directly modified to alter theme behavior, or a theme can be a "child" theme which inherits settings from another theme and selectively overrides features. WordPress users may also create and develop their own custom themes.
WordPress' plugin architecture allows users to extend the features and functionality of a website or blog. As of March 2017, WordPress has over 55,286 plugins available, which enable unique features so that websites can be designed to meet specific needs.
Web developers who wish to develop plugins need to learn WordPress' hook system which consists of over 300 hooks divided into two categories: action hooks and filter hooks.