A DOI (Digital Object Identifier) is an alphanumerical code that permanently links to an item that is located online, like an online journal, web article, news report, or ebook. Links to digital items can move elsewhere or go missing from various sources. A DOI, on the other hand, will always link to the item it specifies.
DOIs are becoming important as a form of citation in some reference styles, especially APA (American Psychological Association). Be sure to check whether your work requires a DOI for your citations!
For APA, the DOI goes at the very end of a citation in your Reference List (the last page). Just type "doi:" and then copy the number and place it at the end.
Here's an example:
Laurance, W. (2010). Better governance to save rainforests. Nature, 467(7317), 789. doi:10.1038/467789d
For additional style formats check the Citation Guide.
On the physical item:
Not all items have DOIs.