What is a DOI? A DOI (digital object identifier) 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.
NOTE: It is regarded as the most important part of the citation because it will accurately direct users to the specific article.
Think of it as a "digital fingerprint" or an article's DNA!
FYI - Updated rules on using DOI (January 2020):
DOI: If a journal article has a Digital Object Identifier (DOI) listed, you will always include this identifier in your reference. You will not have to include the URL of the journal's home page or of the database from which you retrieved the article if a DOI is available.
Online Database: If you viewed a journal article in an online database and it does not have a DOI, you do not include a url for the article or database (p. 299).
Print: If you viewed a journal article in its print format, be sure to check if it has a DOI listed. If it does not, your reference to the article would end after you provide the page range of the article.
Date: When possible, include the year, month, and date in references. If the month and date are not available, use the year of publication.