A child theme is a specially registered WordPress theme that inherits most of its properties from a declared parent theme. A major reason to create a child theme is to be able to update the parent theme without losing desired customizations.
style.css is the only mandatory file in a child theme. The general rule is that child theme files override parent theme files if they exist.
To properly use child themes, you'll need to know how to include the parent theme's style.css; how the WordPress template hierarchy responds to child themes; and under what circumstances you can overwrite existing functions in functions.php.
A child theme is a WordPress theme that depends on a "parent" theme—which could be any other theme already in existence, as long as that theme isn't itself a child theme.
A child theme inherits everything about its parent, and then it only changes what it needs to change. So you could set up a child theme that's identical to its parent...