The Theme Customizer was an exciting new development from WordPress 3.4.  The Theme Customizer allows a user to tweak the theme settings using a WYSIWYG interface, and a real time preview.  The customizer replaces custom theme  option pages, giving developers a whole new set of tools and options to give to users.

With it being a relatively new featuring documentation isn’t as readily available and tutorials are harder to come by, but I found one exceptional tutorial from the Theme Foundation that walks you through a detailed explanation of adding support for the WordPress theme customizer to your WordPress theme, using the theme_mod method.

See the Tutorial here: http://themefoundation.com/wordpress-theme-customizer/