What is a .xml file?

XML is a markup language for structured data that uses opening and closing tags to define every element — verbose but precise, and still foundational to countless systems. The grandfather of structured data. XML was going to be the universal language of the internet — every system would speak it, every document would be encoded in it. That vision was partly realised and partly replaced by JSON. It powers RSS feeds, SOAP APIs, SVG graphics, and the internals of every DOCX, XLSX, and PPTX file (unzip one and look inside). Android layouts are XML. Maven build files are XML. Enterprise software runs on XML. It is not trendy, but it is everywhere and it is not going away.