A child theme will let you use all of the good stuff from a theme you like but dont want to use (i.e. its layouts, design and other features) so that you can have your very own theme without losing the changes if the parent (the theme you like but dont want to use) gets an update.
here is how to create a child theme in its most simple form
first right click on your desktop (on your computer) and select new folder. Name the folder “name-of-your-new-theme-child” and then open it up.
inside the new folder right click once more and select new text document and name it style.css and save that. Once you have the style.css created, simply paste the following code into it. Here goes.
Theme Name: Your theme name Child
Theme URI: http://example.com/
Description: Child theme for the whatever theme
Author: Your name here
Author URI: http://example.com/about/
Template: The Name of the parent theme
@import url("../the parent theme/style.css");
background: url("images/green-sep.png") repeat-x scroll 0 0 #71963C;
border-bottom: 1px solid #47621F;
Now save that file and close it.
Now you can simply take this folder and upload it to your themes directory. You must keep the Original Parent theme in there as this child theme will be riding on the back of its parent theme.
Now once you have it in your theme directory, simply log into your admin panel and go to themes and activate the child theme. When you wish to change something again, simply open the child theme and do your work there.