- XML is a markup language similar to HTML but designed to carry data rather than display it. XML tags are custom defined rather than predefined like HTML tags. - XML documents form a tree structure and have rules around proper nesting of elements, closing tags, and case sensitivity. Core components are elements which can contain other elements, text, and attributes. - XML is used widely on the web for transporting data between applications, simplifying data sharing and platform changes. Common uses include XHTML, RSS feeds, and describing web services.