Actually, style.css depends on how you use it.
For instance, here my child theme style.css beginning
Theme Name: Voodoo Child 2012
Theme URI: http://www.rvoodoo.com/projects/wordpress/wordpress-tip-stop-editing-twentyten-theme-use-a-child-theme
Description: Child theme for the Twenty Twelve theme
Author: Rev. Voodoo
Author URI: http://www.rvoodoo.com
SO in essence, only the child theme css loads. BUT, most of the time we use the @import line which calls to the parent themes css file
If you didn't have that in there, only the style.css of the child would load, you would need to have ALL of the css rules in your child theme to look good
But, we normally use the @import line for ease of customization. What that does is load the parent themes css first, and then you put in any customization you want in your child theme. The child theme css loads after the parent theme - so anything you put into the child theme overrules what is in the parent. That way we can modify the theme!
functions.php is the only file that loads automatically for the parent and child
You don't need one in your child theme, but if you have one, special care must be taken to use it properly.
Here's a tutorial I have about modifying 2012 theme. It has a few examples of modifying a theme using a child theme, specifically using style.css and functions.php
http://vudu.me/child and here are some more tidbits