Do I Need A Child Theme?

If you have made changes to one of the themes that ships with WordPress (Twenty Ten or Twenty Eleven) then when you upgrade you may lose those when you upgrade. Using a child theme protects you against that.

You only need a child theme if you want to modify a theme and want to make sure the parent theme (the one you want to modify) is kept up to date and maintained. The parent theme gets updated without impacting the child theme in it’s own directory.

What counts as modifying a theme?

Making changes to any files within the theme’s directory.

For example, if you changed wp-content/themes/twentyeleven/style.css to modify any of the CSS values then when that theme is updated you would lose your edits too.

By creating a child theme you can make those edit in your child theme and not worry about it. The parent theme gets updated but your changes in the separate child theme directory would remain intact.

It’s a little dated (refers to Twenty Ten) but WP Engineer wrote a thorough post on it.

http://wpengineer.com/2045/understand-wordpress-child-theme/

And of course, there is the Codex article too.

http://codex.ww.wp.xz.cn/Child_Themes