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PURLs (Persistent Uniform Resource Locators) are Web addresses that act as permanent identifiers in the face of a dynamic and changing Web infrastructure. Instead of resolving directly to Web resources, PURLs provide a level of indirection that allows the underlying Web addresses of resources to change over time without negatively affecting systems that depend on them. This capability provides continuity of references to network resources that may migrate from machine to machine for business, social or technical reasons.
Semantic Web entrepreneur David Provost has published a report about the state of business in the Semantic Web and it's a good read for anyone interested in the sector. It's titled On the Cusp: A Global Review of the Semantic Web Industry . We also mentioned it in our post Where Are All The RDF-based Semantic Web Apps?