Content negotiation is a mechanism defined in the HTTP specification that makes it possible to serve different versions of a document (or more generally, a resource representation) at the same URI, so that user agents can specify which version fit their capabilities the best. One classical use of this mechanism is to serve an image in GIF or PNG format, so that a browser that cannot display PNG images (e.g. MS Internet Explorer 4) will be served the GIF version. To summarize how this works, when a user agent submits a request to a server, the user agent informs the server what media types it understands with ratings of how well it understands them. More precisely, the user agent provides an
Accept HTTP header that lists acceptable media types and associated quality factors. The server is then able to supply the version of the resource that best fits the user agent's needs.
So, a resource may be available in several different representations. For example, it might be available in different languages or different media types, or a combination. One way of selecting the most appropriate choice is to give the user an index page, and let them select. However it is often possible for the server to choose automatically. This works because browsers can send information as part of each request about the representations they prefer. For example, a browser could indicate that it would like to see information in German, if possible, else English will do. Browsers indicate their preferences by headers in the request. To request only German representations, the browser would send:
Note that this preference will only be applied when there is a choice of representations and they vary by language.
As an example of a more complex request, this browser has been configured to accept German and English, but prefer German, and to accept various media types, preferring HTML over plain text or other text types, and preferring GIF or JPEG over other media types, but also allowing any other media type as a last resort:
Accept-Language: de; q=1.0, en; q=0.5 Accept: text/html; q=1.0, text/*; q=0.8, image/gif; q=0.6, image/jpeg; q=0.6, image/*; q=0.5, */*; q=0.1
To view the content negotiation by content type and by language, you can use View My User Agent.
Content format 
The user-agent can request the data in a certain format from a web service, such as application/json or application/xml.
See also 
- RFC 2616 — Hypertext Transfer Protocol -- HTTP/1.1 – (Section 12: Content Negotiation)
- RFC 2295 — Transparent Content Negotiation in HTTP
- RFC 2296 — HTTP Remote Variant Selection Algorithm -- RVSA/1.0
- Apache 1.3 Content Negotiation
- Open source PHP content negotiation library (supports wildcards and q values)
- Discussion about XHTML serving with content negotiation and browser concerns requiring this
- The variant button
- Parse Accept-Language to detect a user's language
- Apache 2.0 Content Negotiation Info