Definitions...
nXML
 Extensible Markup Language nXML Document
 File with XML tags nSchema
 Abstract description of the structure
 of an XML document nTransformation (XSLT)
 Extensible Stylesheet Language
 Transformation
XML we have talked about a couple of times already.
It is used to make documents that are often stored as files.
Martin mentioned a DTD before and a Schema is very similar in that it defines the structure of an XML document, much like a one would for a database.  Since XML is hierarchical, a tree structure is described for each file rather than a linear set of fields.
A transformation is just a conversion from one format to another, often between XML and HTML.