What is a DOI?
DOI stands for "digital object identifier;" each DOI identifies and locates a piece of electronic information.
A DOI consists of a string of numbers and letters. They always begin with a 10. The DOI: 10.1111/j.1753-4887.2008.00114.x is for an article about chocolate.
DOIs come into play when creating references for electronic information. Since DOIs are more permanent than urls or web addresses, citation guides (like APA's) prefer them over urls. If you use NoodleBib to create your references, it will ask you for a DOI when appropriate.
Include http://dx.doi.org in front of the doi number, instead of doi (change made summer 2013, according to the latest edition of the APA Style Guide to Electronic References and verified in the official APA Style Blog at http://blog.apastyle.org/apastyle/digital-object-identifier-doi/).
You will normally find DOIs in database information about the article, on the first page of a journal article, or near the copyright notice for the item in question. If you cannot find the DOI on the item itself, you can check CrossRef to see if one exists. Below is an example of a DOI in a database information listing.