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XML is a markup language same as HTML. XML tag you can define your self for storing data. XML store (describe) data, nothing more (either temporary or permanent). XML was designed for describing well formed data. Also XML have some strict rules. That rules follow every XML document.
JSON is another format of expressing data, just like XML. But, JSON is very simpler than XML, and tiny than XML. So, it is becoming popular in the web world to choose the JSON notation over XML since JSON notation are usually shorter, and less data to be transmitted if at all they were to be.