What is a child theme ? Example here for multilingual website

This post to introduce a new series of posts to explain how with few modifications and the way of child theme of twentyten theme containing just modified files, it possible to create a multilingual website. Here, the child theme is named twentyten-xili and his parent is default shipped twentyten.
In functions.php file of child theme some functions where added to improve multilingual features delivered by xili-language plugin. The child style.css is very short and contains just the lines to format languages menu automatically added to header menu and for widget added in sidebar.
Next post : the child theme content… and the advantage of this approach.