A simple non-technical definition:
XML Schemas are similar to forms. And, just like forms they constrain the format and type of acceptable data for XML documents. For example, an XML Schema might require phone numbers to be in the format: "(555) 555-5555."  If you entered 555-555-5555, the entry would be rejected. 

Thus, no matter how large and complex the Schema, the main purpose of an XML Schema is to enforce structure and define valid acceptable data for XML documents. And, as XML Schemas become a common component of B2B, e-government, Web service, and content processing applications, understanding Schemas is more and more critical to professional success.

Our goal is to help our users understand XML Schemas and work with them at every level. At SchemaXplainer, we want to make XML Schemas simple. We invite you to join us.

For those seeking a book on XML Schemas, we recommend Definitive XML Schema. This is by far the best text available on the subject.

• Identify where your data differs from the requirements of your XML Schemas
• Get a handle on how much work is needed to create XML documents from your XML Schemas
• Learn the bits and pieces of your XML Schemas
• Identify key themes and critical elements

• Get a complete overview of all  the components in your XML Schemas
• Identify your XML Schemas' design style and architecture
• Spot inconsistencies in your XML Schemas
• Understand the requirements of your XML Schemas and find areas that need improvement