Google Tag Manager is a new, free service from Google that helps you set up and manage your web site code snippets. Tags, as Google call them, are code snippets like the Google Analytics tracking code and more. You can add any type of tags to the tag manager, not only Google related ones.
I have previously written about how to add Javascript snippets to your Joomla web sites. The tag manager makes this even easier, and you can get away with adding just one piece of code.