Content Negotiation in Web API

One of the standards of the RESTful service is that, the client should have the ability to decide in which format they want the response - XML, JSON etc. A request that is sent to the server includes an Accept header. Using the Accept header the client can specify the format for the response. For example

Accept: application/xml returns XML
Accept: application/json returns JSON

Depending on the Accept header value in the request, the server sends the response. This is called Content Negotiation. 

So what does the Web API do when we request for data in a specific format
The Web API controller generates the data that we want to send to the client. For example, if you have asked for list of employees. The controller generates the list of employees, and hands the data to the Web API pipeline which then looks at the Accept header and depending on the format that the client has requested, Web API will choose the appropriate formatter. For example, if the client has requested for XML data, Web API uses XML formatter. If the client has requested for JSON data, Web API uses JSON formatter. These formatters are called Media type formatters.

ASP.NET Web API is greatly extensible. This means we can also plugin our own formatters, for custom formatting the data.

Multiple values can also be specified for the Accept header. In this case, the server picks the first formatter which is a JSON formatter and formats the data in JSON.
Accept: application/xml,application/json

You can also specify quality factor. In the example below, xml has higher quality factor than json, so the server uses XML formatter and formats the data in XML.
application/xml;q=0.8,application/json;q=0.5

If you don't specify the Accept header, by default the Web API returns JSON data.