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