Child themes prevent overwriting of styling, when the parent theme updates
“A WordPress child theme is a theme that inherits the functionality of another theme, called the parent theme. Child themes allow you to modify, or add to the functionality of that parent theme. A child theme is the safest and easiest way to modify an existing theme, whether you want to make a few tiny changes or extensive changes. Instead of modifying the theme files directly, you can create a child theme and override within.” – WordPress.org
More information and the technical details can be found on the WordPress.org website at http://codex.wordpress.org/Child_Themes .
For the non technical person what you want to understand is that your developer or designer should go the extra and create a custom style sheet to make customization changes to avoid the changes being overwritten.
Try The Plugin Route
This plugin is called One-Click Child Theme. And it is great. With just a couple clicks it will setup an new child theme. And when you are done you can delete the plugin.