URI vs URL vs URN

The term URI is used heavily in the material on XML in our textbook. So far in the course we limited our discussion to URLs. Wikipedia has good discussion of these terms with the relationship of all three depicted in the figure below.
180px-URI_Venn_Diagram.svg
Quoted from that Wikipedia article, "A Uniform Resource Locator (URL) is a URI that, in addition to identifying a resource, provides means of acting upon or obtaining a representation of the resource by describing its primary access mechanism or network "location". For example, the URL http://www.wikipedia.org/ is a URI that identifies a resource (Wikipedia's home page) and implies that a representation of that resource (such as the home page's current HTML code, as encoded characters) is obtainable via HTTP from a network host named www.wikipedia.org. A Uniform Resource Name (URN) is a URI that identifies a resource by name in a particular namespace. A URN can be used to talk about a resource without implying its location or how to dereference it. For example, the URN urn:isbn:0-395-36341-1 is a URI that, like an International Standard Book Number (ISBN), allows one to talk about a book, but doesn't suggest where and how to obtain an actual copy of it."