XML.com FAQs > D. Developers and Implementors (including WebMasters and server operators)
Question:  D.4 What else has changed between SGML and XML?

The principal changes are in what you can do in writing a Document Type Definition (DTD). To simplify the syntax and make it easier to write processing software, a large number of SGML markup declaration options have been suppressed (see the list of omitted features).

An extra Name Start Character is permitted in XML Names (the colon) for use with namespaces (enabling DTDs to distinguish element source, ownership, or application). A colon may only appear in mid-name, not at the start or the end.

This FAQ is from The XML FAQ, maintained by Peter Flynn