A child theme resides in its own directory in wp-content/themes. The scheme below shows the location of a child theme along with its parent theme (Twenty Twelve) in a typical WordPress directory structure:
This directory can contain as little as a style.css file, and as much as any full-fledged WordPress theme contains:
style.css (required)
functions.php (optional)
Template files (optional)
Other files (optional)
Note on the @import rule
There must be no other CSS rules above the @import rule. If you put other rules above it, it will be invalidated and the stylesheet of the parent will not be imported.
Unlike style.css, the functions.php of a child theme does not override its counterpart from the parent. Instead, it is loaded in addition to the parent’s functions.php. (Specifically, it is loaded right before the parent’s file.)
But, when creating child themes, be aware that using add_theme_support('post-formats') will override the formats as defined by the parent theme, not add to it.
A child theme can override any parental template by simply using a file with the same name.