What is XML? The tutorial at W3Schools:
http://www.w3schools.com/xml/default.asp

What is JSON? The tutorial at W3Schools:
http://www.w3schools.com/json/

Comparison between the two:
http://stackoverflow.com/questions/2636245/choosing-between-json-and-xml
http://stackoverflow.com/questions/15154049/xml-vs-json-which-one-is-better-for-storing-small-chunk-of-data