URIs : How to find the ontologies behind them

A question about URIs:

The URI for a class does not have to point to a particular resource on the web, it just has to be unique.  An ontology describing that entity may be located somewhere else entirely.

If an ontology refers to this class by its URI, how is additional, or even primary, information about that class supposed to be found?  

Any help is appreciated.

Eli Israel, Director
www.SemanticWorld.Org