What is a WordPress Plugin? — a practical guide to What is a WordPress plugin with clear examples you can reuse in real projects.
WordPress Plugin Development Series (2/95). Prefer one article? Read the complete WordPress plugin development tutorial.
Short description
A plugin is a PHP (and JS/CSS) package that hooks into WordPress actions/filters to add features — themes change design; plugins change functionality.
Plugin vs theme
Theme → look & layout
Plugin → features & behavior
Best practice: keep logic in plugins