https://developer.mozilla.org/en-US/docs/Web/HTTP/Access_control_CORS

Cross-site HTTP requests are HTTP requests for resources from a different domain than the domain of the resource making the request.  For instance, a resource loaded from Domain A (http://domaina.example) such as an HTML web page, makes a request for a resource on Domain B (http://domainb.foo), such as an image, using the img element (http://domainb.foo/image.jpg).  This occurs very commonly on the web today — pages load a number of resources in a cross-site manner, including CSS stylesheets, images and scripts, and other resources.

Cross-site HTTP requests initiated from within scripts have been subject to well-known restrictions, for well-understood security reasons.  For example HTTP Requests made using the XMLHttpRequest object were subject to the same-origin policy.  In particular, this meant that a web application usingXMLHttpRequest could only make HTTP requests to the domain it was loaded from, and not to other domains.  Developers expressed the desire to safely evolve capabilities such as XMLHttpRequest to make cross-site requests, for better, safer mash-ups within web applications.

The Web Applications Working Group within the W3C has recommended the new Cross-Origin Resource Sharing (CORS) mechanism, which provides a way for web servers to support cross-site access controls, which enable secure cross-site data transfers.  Of particular note is that this specification is used within an API container such as XMLHttpRequest as a mitigation mechanism, allowing the crossing of the same-domain restriction in modern browsers.  The information in this article is of interest to web administrators, server developers and web developers.  Another article for server programmers discussing cross-origin sharing from a server perspective (with PHP code snippets) is supplementary reading.  On the client, the browser handles the components of cross-origin sharing, including headers and policy enforcement.  The introduction of this new capability, however, does mean that servers have to handle new headers, and send resources back with new headers.

This cross-origin sharing standard is used to enable cross-site HTTP requests for:

• Invocations of the XMLHttpRequest API in a cross-site manner, as discussed above.
• Web Fonts (for cross-domain font usage in @font-face within CSS), so that servers can deploy TrueType fonts that can only be cross-site loaded and used by web sites that are permitted to do so.
• WebGL textures.
• Images drawn to a canvas using drawImage.

This article is a general discussion of Cross-Origin Resource Sharing, and includes a discussion of the HTTP headers as implemented in Firefox 3.5.