[DonRotolo]

XML stands for eXtensible Markup Language. It is a cousin of HTML, and both are 'plain text' formats. You can open and edit it in Notepad. MS Word has an XLM interpreter, but if your XML is non-standard it will not be any fun. Notepad is very unintelligent but won't cause you any grief either.

XLS is a spreadsheet format used in MS Excel. It is a binary file, most definitely not 'plain text', and as unrelated to XML as MP3 is unrelated to PDF. Don't use Excel to open or edit XML files.