In the present world, where companies are providing SAAS (Software Applications As Services) over the Web, and applications are mashing-up with other applications... in such scenarios everyone wants to make applications, to be able to exchange data with other applications seamlessly and as fast as it can.
Seamless integration of data was made possible by XML, but it seriously lacked the speed. Over and above it, XML required specific Serialization and De-Serialization of content. This is were JSON has the real advantage over XML and it doesn't compromise on the other advantages of XML.
More information on JSON can be found at the following links: