XSD is a W3C recomedation that specifies how to describe the elements in XML document
XSD specifies data types, order and arity of elements in XML file.
                Main components of XSD file are:
                - Element declaration - declares properties of elements (names and namespaces)
                - Attribute declarations - declares properties of attributes
                - Simple and complex types:
                - - XSD provides 19 simple data types
                - - More complex types are declared using simple types and relationships