XML Processors

An XML Processor is a piece of software, either an application or a 
library, that can process XML.  A good example of an XML processor 
is XML document validation software.  There are a number of such 
packages available for free and for sale on the Internet.  Such applications 
can be used to validate the contents of an XML document and make 
sure that it is well-formed.  A well-formed document is one that adheres 
to the rules of XML and any associated DTD’s.

Other good examples include XML document viewers and XML 
document code libraries that can be used by software that you create 
to manipulate XML documents.