A DOI, or digital object identifier, is like a social security number for a document online. It’s a unique and permanent identifier that will take you straight to a document no matter where it’s located on the Internet.
You can Google a DOI to find an article, although you may still have to sift through search results. To go straight to the source, you can also consult a DOI resolver, such as the one supplied by CrossRef.org.
The DOI should be listed prominently on the first page of an article, whether in print or online. If you’re working online, copy and paste it into your reference list, to avoid transcription errors. Do not alter the alphanumeric DOI string in any way.