WordPress is open source PHP software using MySQL, and the most widely used content management system on the web by a large margin. It is both the most common source and the most common destination when content moves.
How it models content
Almost everything is a post. Posts, pages, attachments and menu items all live in the same table, separated by a post type column. That design is why WordPress is flexible, and why an export can contain more than you expect.
| Posts | Dated entries, usually the blog |
| Pages | Undated, and can be nested to form paths |
| Custom post types | Anything a theme or plugin defines |
| Categories and tags | Taxonomies, which can also be custom |
| Media | Files on disk, with a database record each |
| Post meta | Arbitrary extra fields attached to any post |
Where the difficulty sits
Post meta. Titles and body content are simple, but plugins store their data as meta attached to posts, and there is no standard for what goes in there. Two sites running the same plugin can hold the same information in different shapes.
Anything built with a page builder is harder still, because the layout is stored as markup or shortcodes inside the content field. Move that content elsewhere and you get the builder’s markup rather than clean text.
Current status
Actively developed, on a regular release cycle, with by far the largest theme and plugin ecosystem of any CMS.