A DOI (digital object identifier) is a unique number assigned to an article to help with online article retrieval.
Think of it as a barcode, book ISBN, or UPC for information.
Most citation styles require that you include the DOI at the end of your references for both print and electronic articles.
There are three places you can look for an article DOI*:
On the database search results page within the information about the article;
On the first page of the full text of the article;
On the search results page after using crossref.org's free DOI lookup tool (linked below).
*DOIs can be assigned to an article at any time. Don't assume that an older article won't have a DOI.
APA guidelines state that if there is no DOI available, then at the end of your citation, include “Retrieved from” and the web address for the journal homepage*.
*Note: Not the direct web address for the article.