This document discusses Uniform Resource Identifiers (URIs) and how they are used to uniquely identify resources on the semantic web. It provides examples of URIs used by databases like DBPedia, Freebase, and Geonames to identify things like New York City. It explains that while a URI uniquely identifies a resource, accessing that URI may provide a description of the resource instead of the resource itself. URIs are a key component of the semantic web as they allow different data sources to link to each other through references to the same unique URI identifiers.