XML (Extensible Markup Language) is a markup language similar to HTML, but without predefined tags to use. Instead, you define your own tags designed specifically for your needs. This is a powerful way to store data in a format that can be stored, searched, and shared. Most importantly, since the fundamental format of XML is standardized, if you share or transmit XML across systems or platforms, either locally or over the internet, the recipient can still parse the data due to the standardized XML syntax.

For an XML document to be correct, the following conditions must be fulfilled:

Document must be well-formed.
Document must conform to all XML syntax rules.
Document must conform to semantic rules, which are usually set in an XML schema or a DTD.