XSD stands for XML Schema Definition, which is a recommendation by the World Wide Web Consortium (W3C) to describe and validate the structure and content of an XML document. It is used to define the elements, attributes, and data types that an XML document can contain, and it is primarily used to verify if each element, attribute, or data type in the document matches its description. XSD is similar to earlier XML schema languages, such as Document Type Definition (DTD), but it is a more powerful alternative as it provides greater control over the XML structure. XSD files are stored in XML file format and can be opened or edited in any text editor such as Microsoft Notepad, Notepad++, or Microsoft XML Notepad. XSD files ensure that the data entered follows the same structure as defined in the file, and errors are caught, resulting in appropriate XML validation.