James Madison University Librarian defines DOI (2017).
DOI = Digital Object Identifier
Source: Publication Manual of the American Psychological Association, 7th edition
A DOI URL looks like this ----- https://doi.org/10.1007/s10754-013-9127-4
A general DOI number is -- 10.1007/s10754-013-9127-4
Note this change is backwards compatible and http://dx.doi.org/ and http://doi.org/ will continue to work indefinitely.
Two links that help with verifying DOIs:
Here are a couple of resources explaining what DOI is, when and why created.