Libxml2 is the XML C parser and toolkit developed for the Gnome project, but is usable outside of the Gnome platform as well.
XML itself is a metalanguage to design markup languages, i.e. text language where semantic and structure are added to the content using extra "markup" information enclosed between angle brackets. HTML is the most well-known markup language.
Though the library is written in C a variety of language bindings make it available in other environments.