While Microsoft has chosen to use XSD as the schema language throughout its product line, there are at least two other strong schema languages in regular use today. Both Document Type Definitions (DTDs) and RELAX NG (pronounced "relaxing" but no longer an acronym) are much simpler than XSD, and both offer features that go beyond those in XSD, though in different ways. Developers who need or prefer to integrate DTDs or RELAX NG with Microsoft Office can do so by converting these schemas into XSD with Trang, a very simple but powerful translation tool.
If you need a more detailed explanation of DTDs, any introductory XML book should provide a lot of information. For a more thorough explanation of RELAX NG, see Eric van der Vlist's RELAX NG (O'Reilly).
XML.com also has a number of excellent articles on RELAX NG and RELAX NG compact syntax that go beyond the coverage in this appendix. To test RELAX NG schemas and perform conversions of either DTDs or RELAX NG schemas into XSD, you'll want to download James Clark's Trang package from http://www.thaiopensource.com/relaxng/trang.html. Trang requires that you have a Java runtime installed, but includes all the Java classes it needs in two Java JAR packages.