URL is an URI which identifies a resource by location.
Resource representation is a view of resource's state. This view is encoded in one or more transferable formats, such as XHTML, Atom, XML, MP3 ...
URIs associate resource representations with their resources
a) So I assume URI identifies a resource and not resource representation?
b) I've read that relationship between an URI and resource representation is one to many. Assuming we're talking about URL, how can a single URL address more than one resource representation?