The Washington Area SGML/XML Users' Group will meet at the American Geophysical Union (2000 Florida Ave, NW) at 7:00 PM. There are two topics: XML W3C Schema Basics (Karanjit Siyan, National Library of Medicine) and Justice Community XML Standards (Diane Lewis, Dept of Justice).
XML Schema is the standard approach to specifying the contents of an XML document. XML folk like to talk about two concepts: well-formed and valid. A well-formed XML document is any document that complies with the XML standard. You can use any tags you like, as long as the angle brackets are in the right order. A valid XML document is one that complies with a DTD or schema. If you want to learn about schemas, a really good starting point is the W3C's primer.