Definition
WordPress is an open-source publishing system for building and managing websites with themes, plugins, and content management. The official About page records that it started in 2003, when Mike Little and Matt Mullenweg forked b2/cafelog, and that it is built on PHP and MariaDB and licensed under the GPLv2. See the official About page.
How it works
The software is downloaded from wordpress.org and installed on your own server: content is managed through an admin dashboard, appearance is controlled by themes, and features are added through plugins. The project describes its mission as “democratize publishing”; the GPL preserves the freedoms to run, study, change, and redistribute the software. According to the official page, people with limited technical experience can use it out of the box, while technically minded users can customize it extensively.
Usage
It covers publishing scenarios from blogs to personal and business sites. Because it is built on PHP and a database, it needs a server environment that supports both. A large ecosystem of third-party themes, plugins, and tutorials surrounds it; actual functionality depends on the components chosen.
Related and confusable terms
WordPress.org and WordPress.com have similar names: the official comparison explains that both run the same software and the main difference is hosting — .com is a service that includes hosting, while .org is the open-source software you deploy yourself (see the official comparison). This entry is about the wordpress.org open-source software.
Episode discussion
In Weekly #007, in the chapter “安全风险集中爆发：漏洞、证书与补丁” (Chinese transcript), Guizang, discussing a vulnerability affecting PHP, says some countries took services offline first, “for example WordPress — I have many WordPress sites”, noting that it is built on PHP. This is a speaker’s account within a security discussion; specifics belong to official security advisories. See the chapter.
Frequently asked questions
Do I need to write code to build a site with WordPress?
Not necessarily. The official About page describes users with limited technical experience using it out of the box; code comes in for deep customization.
What is the difference between WordPress.org and WordPress.com?
Same software, different hosting: .org is the self-deployable open-source software, .com is a hosted service. The official comparison is at the com-vs-org doc.