An XSD file, short for XML Schema Definition, is a file format used to define the structure and content of XML documents. It acts as a blueprint or schema that specifies the elements, attributes, data types, and relationships that are allowed within a valid XML document. XSD files are written in XML themselves, providing a standardized and machine-readable way to describe the grammar of an XML language. They are crucial for ensuring data integrity and consistency when exchanging XML data between different systems or applications. By validating an XML document against its corresponding XSD schema, developers can verify that the document conforms to the defined rules and constraints, preventing errors and ensuring interoperability. XSDs offer more advanced features than older DTDs (Document Type Definitions), including support for data types, namespaces, and complex content models. They are widely used in web services, data integration, and other applications where structured data exchange is essential.