DOI

A digital object identifier (DOI) is a permanent, unique string of characters that identifies specific digital content such as an online article, dataset, image, etc.). Ideally, when a DOI is assigned (by a publisher, repository host, etc.), there is metadata and a URL which is associated with the DOI which makes the DOI 'resolvable' (point to the digital object or to a web resource that gives you more information about the digital object). 

All DOI numbers begin with a 10 and contain a prefix and a suffix separated by a slash. The prefix is a unique number assigned to organizations; the suffix is assigned by the the organization.

Examples:

10.1159/000330840
10.1289/ehp.1003206

DOIs can often be used in citations in lieu of urls.

• AMA Style Guide
• APA Style Guide

• Usually located on the first page of an electronic journal article, near the copyright notice or near the author's address.
• May also be found in the Full Record Display of an article in a database, with the field labeled DOI.
• Use this format for DOI in your references: doi:xxxxxxxxxxx

To make the DOI resolvable, add the DOI to the end of this url:

http://dx.doi.org/

For example,  doi: 10.1016/j.jalz.2017.09.010

url: http://dx.doi.org/10.1016/j.jalz.2017.09.010