URLs Versus URIs

Having discussed XLinks and XPointers, I should also mention that the XML specification expands the idea of standard URLs (uniform resource locators) into URIs (uniform resource identifiers).

URLs are well understood and well supported on the Internet today. On the other hand, as you'd expect given the addition of XLinks and XPointers to XML, the idea of URIs is more general than simple URLs.

URIs represent a way of finding resources on the Internet, and they center more on the resource than the actual location. The idea is that, at least in theory, URIs can locate the nearest mirror site for a resource or even track a document that has been moved from one location to another.

In practice, the concept of URIs is still being developed, and most software will still handle only URLs.

URI: Formal Definition

You might want to look up the formal definition of URIs, which you can find in its entirety at www.ics.uci.edu/pub/ietf/uri/rfc2396.txt.