.XSD is a file extension used for an XML Schema. This is a pretty complex specification, and if you're going to understand the schema you've been given, you're going to need to read a book on the subject. A number of XML editors such as Stylus Studio and XML Spy will let you do schema-based document editing, but that's not going to help you much unless you understand the schema first.
A schema is not intended as a guide for document authors. You should go back to the people who sent you this file and ask for some author-oriented documentation. The schema can help you validate that your document conforms to the rules, but it's not a user manual.
Michael Kay
http://www.saxonica.com/
Author, XSLT Programmer's Reference and XPath 2.0 Programmer's Reference