SharePoint's Document ID service, available with the pay versions of SharePoint, creates unique IDs for documents that are prefixed with text the Site Collection Administrator defines. What's more is that the ID assigned to the document creates a fixed URL that can be used to link to the document even if it is moved to another library or site. The Document ID service can only be used on documents and cannot be used on list items.
The Document ID service is enabled at the Site Collection level by activating a feature. Once the service is activated the Site Collection Administrator can define the prefix string to be used. SharePoint uses a Timer Job, Document ID assignment job, to set the IDs on existing documents once the Document ID Service has been activated. The Timer Job runs every 24 hours by default so existing documents will not have the IDs until the job has run. A Farm administrator can manually run the job to have the IDs created sooner. New documents will have IDs created when the document is saved to the library.