XML stands for Extensible Mark-up Language which is similar to HTML. It improves the functionality of the Web by letting you identify your information in a more accurate, flexible and adaptable way. The main objective of XML is to carry the data; however the main objective of HTML is display the data. Unlike HTML, the tags here are not predefined; one most define their own tags. It is self-descritive and a W3C recommendation.
It is extensible because it is not a fixed format like HTML (which is a single, predefined markup language). Instead, XML is a metalanguage - a language for describing other languages - which lets you design your own markup languages for limitless different types of documents. XML can do this because it's written in SGML, the international standard meta languages for text document markup (ISO 8879).
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