script src="//ajax.googleapis.com, where is the http?

Basically it allows the browser to determine whether to use http:// or https:// based on the protocol that your page is using. If your page is on https then it will use https to get the scripts. Because if you hard code to say http and you page is using https then the browser will throw errors telling the user that the site might not be safe for instance. Bottom line always use relative URL for scripts, images and all resources and don't hard code the http protocol.