What is the difference between XML and XSD?

Question

What is the difference between Extensible Markup Language (XML) and XML Schema (XSD)?

Answer 1

Take an example

  
     Y
     12
  

and design an xsd for that:

What isn't possible with XSD: would like to write it first as the list is very small
1) You can't validate a node/attribute using the value of another node/attribute.
2) This is a restriction : An element defined in XSD file must be defined with only one datatype. [in the above example, for appearing in another node, datatype cannot be defined other than int.
3) You can't ignore the validation of elements and attributes, ie, if an element/attribute appears in XML, it must be well-defined in the corresponding XSD. Though usage of allows it, but it has got its own rules. Abiding which leads to the validation error. I had tried for a similar approach, and certainly wasn't successful, here is the Q&A

what are possible with XSD:
1) You can test the proper hierarchy of the XML nodes. [xsd defines which child should come under which parent, etc, abiding which will be counted as error, in above example, child_two cannot be the immediate child of root, but it is the child of "parent" tag which is in-turn a child of "root" node, there is a hierarchy..]
2) You can define Data type of the values of the nodes. [in above example child_two cannot have any-other data than number]
3) You can also define custom data_types, [example, for node , the possible data can be one of the 12 months.. so you need to define all the 12 months in a new data type writing all the 12 month names as enumeration values .. validation shows error if the input XML contains any-other value than these 12 values .. ]
4) You can put the restriction on the occurrence of the elements, using minOccurs and maxOccurs, the default values are 1 and 1.

.. and many more ...