A transformation is an operation that takes an input, performs an action on it, and returns an output. Examples of XML transformations include XQuery and XSLT. Transformation output can be a finished product - XSLT that creates an HTML report, for example - or it can be something that is passed along to another operation for additional processing - XQuery that specifies FO post-processing of the XML it generates, or output passed to an XML Schema for validation, for example.