Child Themes « WordPress Codex

Child Themes « WordPress Codex: "A WordPress child theme is a theme that inherits the functionality of another theme, called the parent theme, and allows you to modify, or add to, the functionality of that parent theme. This article shows how to create a basic child theme and explains what you can do with it. As an example parent theme it uses Twenty Ten, the new default theme in WordPress 3.0.

Making a child theme is very simple. Create a directory, put a properly formatted style.css file in it, and you have a child theme! With a little understanding of HTML and CSS, you can make that very basic child theme modify the styling and layout of a parent theme to any extent without editing the files of the parent theme itself"