An identifier for an object in a given domain or namespace, can be presented as a URL or a URN.
A typical representation of a URI can be found in your every-day website address.
http://rdfblog.com/blog/URIs-URLs-URNs
Originally URLs were designed as an informal concept for identifying objects based on location within a domain, but also proved to become quite ideal for linking resources.
URNs can identify objects based on name, for example;
urn:isbn:n-nn-nnnnnn-n
"http" and "urn" are both URI schemes, in a list of 84 that are publically available, around 30 are registered.
However, this list has now been retired, but fortunately I've discovered a wiki that covers URI registration.
Many of the references to Objects, Subjects and Predicates in RDF use URIs to represent their nodes, so hopefully this post helps to clear the air.