A DOI, or Digital Object Identifier, is a unique string of numbers and letters that permanently identifies an article, or other work, and links to it on the web. In short, a DOI is a permanent or persistent identifier.
DOIs are intended to make information retrieval easier.
DOIs identify with the content rather than a location. If the item changes location, the publisher is responsible for pointing the DOI to the new location. If this updating does not take place, you canend up with a dead DOI.
DOI FORMATS -- ACTIVE or INACTIVE
When you locate an article, the DOIs may be formatted as active or inactive.