The fundamental concepts that form the base layer of the Symantic Web stack are universal, standardized identification (URI) and a universal, standardized character set (Unicode).
Uniform Resource Identifiers (URIs) are formulated in a standard syntax that is used to support information exchange in the Internet, particularly on the World Wide Web (Web). Their aim is to uniquely identify objects (or resources) located in any directory on any machine in the network accessed via a specified access method. There are two types of URIs, URLs and URNs.
A URL identifies a resource via a representation of its primary access mechanism, e.g., its network location. Here is an example of how a URL is formatted:
The name of the protocol to be used (in this example) is http. The address can be either an IP or a domain name which in our example is the domain name www.somedomain.com. The hierarchical path describes the location of the resource on the machine, in this case the folder abc. The name of the resource is index.html.
A Uniform Resource Name (URN) is a locally unique and persistent identifier of a resource in a namespace that determines the syntax for the URN identifier. Whereas a URL specifies the location of the resource, a URN specifies its name independent of location. URNs are coded by the prefix "urn:" and are used to define subspaces of identifiers, called namespaces.