The acronym DOI might be new or confusing for some. New versions of some citation styles now take into account whether or not a DOI is given for a particular article. Here is a definition of DOI from the International DOI Foundation:
"The Digital Object Identifier (DOI) System is for identifying content objects in the digital environment. DOI names are assigned to any entity for use on digital networks. They are used to provide current information, including where they (or information about them) can be found on the Internet. Information about a digital object may change over time, including where to find it, but its DOI name will not change."
Here's an example of where you can locate the DOI for an article in Academic Search Complete: