XSL is the stylesheet (eXstensible Stylesheet Language) that is used to display the XML in a similar manner to that of plain HTML.
XSLT or XSL transformations is the markup vocabularly of the XSL - you can find the most current version on http://www.w3.org.
In order to bring the XML and XSL together (the XSLT is written in the XSL) you need a PARSER (i use an ASP one)
A reasonable website to look at is http://www.xml101.com although i'm sure there are better ones out there...