Someone has been thinking logically. It’s the best description of WordPress that I’ve seen in a while:

A WordPress theme is a collection of PHP and CSS files that change the appearance of your site without changing the content, which is stored in a database. This allows you to easily change the presentation of your website by switching themes, while the content remains.

To use a metaphor, the Theme system provides a “skin” your website while WordPress is providing the “bones” of the site in its underlying code.

Read the rest of the article here:

http://www.hyperarts.com/blog/intro-wordpress-theme-frameworks-child-themes/