XSD, or XML Schema Definition, is a language used to define the structure and rules of an XML document. It specifies the elements, attributes, and data types that can appear in the document, ensuring that the data adheres to a predefined format. This helps in validating the content of XML files, making sure they are both correct and consistent. By using XSD, developers can create more robust applications that rely on XML for data interchange. It allows for better data integrity and easier integration between different systems, as it provides a clear contract for how data should be structured and validated.