What do HTTP URIs Identify?
http://www.w3.org/DesignIssues/HTTP-URI.html
However, the lack of very concise logical definition of such things had not been a problem, until the formal systems started to use them. There were no formal systems addressing this sort of issue (as far as I know, except for Dan Connolly's Larch work [@@]), until the Semantic Web introduced languages such as RDF which have well-defined logical properties and are used to describe (among other things) web operations.