As it is widely known, AJAX Requests are only possible if port, protocol and domain of sender and receiver are equal. [...] Having this cleared out, we will cover ways around this restriction.

He covers three main approaches to allowing these cross-domain requests (and some of the security implications that can come with them):

• CORS (Cross Origin Resource Sharing)
• JSONP (Javascript with a local domain callback)
• Iframes

He also briefly mentions things like window.postMessage (HTML5) and the use of a backend script to proxy a request into your application's local code.