Considerable confusion exists about XML Schema. Simply put, XML Schema represent another, more flexible way to accomplish the same functions as DTDs. Just like the DTD, the XML schema is an external document referenced from the XML document. In terms of learning objects, XML Schema do very little additional, useful things, as compared with DTDs.
Where XML Schema do come into their own are in validation of numerical data. For example, if you are passing instructions to a machine that cuts precision parts from blocks of steel in XML format, it would be handy to have that data automatically validated before it is handed to the machine, since accidentally sending the wrong numbers can ruin not only the production piece but the very expensive machine as well.
It also is true that most of the document-handling software has been developed in an XML-with-DTD environment. If, in the future, as some forecast, XML Schema replace DTDs, there will be simple tools to convert DTDs to equivalently behaving XML Schema. For the time being, in regard to learning content management, it is not necessary to pay any attention to schema[10].