What is a digital object identifier, or DOI?*
A digital object identifier (DOI) is a unique alphanumeric string assigned by a registration agency (the International DOI Foundation) to identify content and provide a persistent link to its location on the Internet. The publisher assigns a DOI when your article is published and made available electronically.
All DOI numbers begin with a 10 and contain a prefix and a suffix separated by a slash. The prefix is a unique number of four or more digits assigned to organizations; the suffix is assigned by the publisher and was designed to be flexible with publisher identification standards.
We recommend that when DOIs are available, you include them for both print and electronic sources. The DOI is typically located on the first page of the electronic journal article, near the copyright notice. The DOI can also be found on the database landing page for the article.
For more on DOIs, see Electronic Sources and Locator Information (PDF) and the DOI category of the APA Style Blog.
*The web page for this information is located on the APAStyle web site: http://www.apastyle.org/learn/faqs/what-is-doi.aspx