What is a DOI?
DOI= Digital Object Identifier
A DOI identifies an individual article (like a serial number) such that a permanent URL can be created.
Example of DOI: 10.1000/182
The first part before the / is the publisher portion of the number. The second part points to the journal and article specifically.
Some citation styles require a DOI. Do you have a DOI and nothing else? This is where you can go to resolve the DOI: http://dx.doi.org/
How to cite a DOI in APA style: http://owl.english.purdue.edu/owl/resource/560/10/