Extensible Markup Language, abbreviated as XML, is a markup language used to define documents with a standard format that is human-readable as well as machine-readable through the use of tags. It is a hardware and software-independent tool for storing and transporting data.

In simple words, it is a simpler subset of the SGML, a standard to craft a document structure. It uses a Document Type Definition (DTD) to describe the data formally. XML data is self-defining, suggesting that the structure of the data is embedded with the data, hence there is no need to pre build the structure to store the data.