The data representation standards address the problem of heterogeneous data representation i.e. how to represent data in a format that is equally understood by everyone. This problem is shared by all Web Services standards.

XML is a text-based tag language that allows the representation of data in a structured and self-describing way. A document that conforms to XML syntax is called well-formed. XML is used as canonical format for data exchanged in Web Services.

XSD is a grammar to define XML document schemas, including: elements, attributes, order, cardinality, data types and default values. A well-formed document that conforms to a XSD is called valid.

XPath is a general-purpose syntax to reference elements or attributes inside a XML document.