XML was the core language of Web 1.0. It formed the foundation for nearly everything you read with your browser. You might not know this, though, because of the great number of languages and acronyms you find. Web pages were written in XHTML, news feeds are written in RSS, and many applications still communicate to each other with XML-RPC.
Though nowadays most folks use JSON or a binary protocol. XML was used everywhere, and that turned out to be a little too much.