Resource Description

CrossRef is a freely available service that aids in identifying the DOI (digital object identifier) that is a required element by some bibliographic styles (such as APA). "A DOI is an alphanumeric name that identifies digital content, such as a book or journal article. The DOI is paired with the object's electronic address, or URL, in an updateable central directory, and is published in place of the URL in order to avoid broken links while allowing the content to move as needed." CrossRef provides several different means for identifying a needed DOI.