DOI stands for Digital Object Identifier. They’re used as a way to link persistently to an article so that – provided they are kept up to date – any link or reference to a DOI should take a user to where the article currently resides online. They’re useful for things like citation metrics, but also as a way to prevent or combat dead links. From Crossref:
[A DOI is] a unique alphanumeric string assigned to a digital object – in this case, an electronic journal article or a book chapter. In the Crossref system, each DOI is associated with a set of basic metadata and a URL pointer to the full text, so that it uniquely identifies the content item and provides a persistent link to its location on the internet.
For more information on the DOI itself, which is a NISO standard syntax, please visit the International DOI Foundation website at www.doi.org. For details on use of the DOI within Crossref, please see the “How Crossref Works” page.
See more at: http://www.crossref.org/01company/16fastfacts.html#sthash.aSBsVFta.dpuf
The key to this statement is that a DOI is unique. Any article should only have one unique combination of a DOI prefix and suffix. Articles should also, ideally, only have one DOI each.