Child theming is a safe way to modify a theme. A child theme inherits a "parent themes" styles and then can selectively override. This alternative to "destructive" editing allows for safe updating of the parent theme and provides an easy way to undo theme changes.
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. Quote from Codex
A child theme appears as a unique theme in the admin UI under "Appearance" » "Themes" settings and is activated instead of the parent theme (which must also be installed to activate the child theme).
It's style.css file uses a standard WordPress Theme header but includes the additional line:
parenttheme is the folder in which its parent theme resides.
Once the child theme is setup and activated, it inherits all of the parent theme's files unless a file of the same name resides in the same location/dir inside the child theme folder. In that case, the file in the child theme folder is used instead. 1)
1) The only exception to this rule is that both the parent and child themes'
functions.php files are run, with the parent theme's file running after the child themes