are a means of embedding additional information
within a document, beyond that contained in the "pure" document content
The term originates from the days when computer documents were prepared from existing paper documents. The first part of the process was to take a hand- or type-written page and manually write on it (i.e. mark it up with) the appropriate formatting commands. These, along with the document content, would then be entered onto the computer.
IBM is responsible for some early work on generalised markup languages (GMLs), including its mainframe product SCRIPT. SGML - standard generalised markup language - is derived from this early work.
SGML derivatives (and, indeed, GML and other IBM markups) depend on a DTD, or Document Type Definition. This defines the markup "tags" and how they can be used. In most cases, the presentation of the information is also specified in the DTD definitions of the tags. However, XML relies on a separate means of supplying layout information - style sheets.