XSLT and XPath
In Umbraco an important part of generating web page content is based on Macro's written in XSLT to extract information from the XML representation of the website content. XSLT in turn uses XPath to identify the parts of the XML-document to work on.
Below follows a short introduction to XSLT and XPath and also a reference to the both based on the information in XML in a nutshell (2nd german edition) by E.R. Harold and W.S. Means.
XSLT
XPath
Reference documents