Content negotiation mean get data in a format that client need..

“content negotiation is the mechanism that is used for serving different representations of a resource at the same URI“

We need to tell the service by defining in Request header.For example we want a service to return data in XML format and Json at another time with same service. So to achieve this we will send corresponding Content-type as below..

For Json

Content-type: application/json

For XML

Content-type: application/xml

When we send these Content-type web api checks the request header and response accordingly with the help of MediaFormatter in .Net.

Similarly getting desired image for mate,document formats,language etc. Can be considered Content negotiation.