Hello,
XML - Extended markup language . See
this.
XSD - XML Schema definition - This defines the XML format. Earlier it used to be DTD and now the format as well is defined using XML structure.
XLS - MS Excel spreadsheet file.
I am not sure why you must have clubbed the XLS along with XML and XSD. Apart from "X" there is no commanility in between them.
Regards,
Amit