You can find some simple explanation and few examples of XSLT in this document. XSLT is very useful in Sterling Integrator. It can translate XML source into another XML, HTML or simple text.
A typical usage of XSLT translation is preparing message for mail body with information about successful processing or error notification.
XSLT can take either Primary Document or Process Data as a source/input for translation.
 

Find the XPath explanation and examples in the following documents.

This document describes only some general information about XML. It is very short description, for better understanding you should find some other resources.