JSON (Javascript Object Notation) is a lightweight data format which has far less overhead then xml and is supported by a wide variety of programming languages. Detailed information on the specification can be found here (http://www.json.org/). In a nutshell JSON uses a combination of braces, quotes colons and commas to delineate fields. When returning large datasets this is a substantial savings over the open and close tags of xml.

Here is an example return from a database.