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 (March 2017):
(Above information from The APA blog on DOI. This is APA's official blog and their staff provides excellent examples on APA formatting/citations.)