- A child theme allows customizing a parent theme without modifying the parent theme files directly. It is dependent on the parent theme and only requires a style.css file. - Child themes make customizations future-proof, allow building on a parent theme's codebase, and can override or add functionality to the parent theme. - The child theme style.css and functions.php files override the parent theme files. Other template files in the child theme are used if they exist, otherwise the parent theme files are used.