This is the basic explanation of what SQL/XML functions provide in Oracle (since 9iR2) Database as per SQL 2003 standards.

• XMLElement takes an element name, an optional collection of attributes for the element, and arguments that make up the element’s content and returns an instance of type XMLType.
• XMLForest converts each of its argument parameters to XML, and then returns an XML fragment that is the concatenation of these converted arguments.
• XMLConcat takes as input a series of XMLType instances, concatenates the series of elements for each row, and returns the concatenated series. XMLConcat is the inverse of XMLSequence.
• XMLAgg takes a collection of XML fragments and returns an aggregated XML document

I am going to write a couple of examples on how to use these functions in next day or two.