The basis of XSL is the template rule, which establishes how the original XML element is to appear in the output document. Each template rule is made up of a pattern and a template. The template specifies what is to happen to the pattern in order to produce the relevant part of the results tree. If you consider the XML source document the raw material and the final HTML document the finished product, then the construction rule represents the manufacturing process. Basically, these rules turn the raw data of XML into viewable information.
Patterns are strings that match the names of elements in the XML document.
They specify the elements to which you want to apply the template. Several other types of pattern matching are also available through XSL, but basic and ancestry are the most common. Find out how.