Together with attributes, tags are used to structure information. The XML syntax has rapidly imposed itself as a popular format for exchange of information on the web. Around this syntax, a large set of companion standards and tools are being developed. Mark-up refers to annotations in the margin or in the body of a document for styling, etc. For example, when you review a document and note in the margin that the title should be in bold, you are marking up the document.
Tags are delimiters for XML elements.
XML elements are enclosed between opening and closing tags. It is EDI with XML, or XML with EDI - depending on the perspective. This is the Extensible Style Sheets Language developed by W3C primarily to control the way information is presented on screen. XSL also provides some key document-handling facilities beyond styling. For instance, transformations, re-arrangement of elements, extract of a table of contents from a document.