Q. What is DOI? I saw it on a periodical article.

A digital object identifier (DOI) is a unique alphanumeric string assigned by a registration agency (the International DOI Foundation) to identify content and provide a persistent link to its location on the internet. The publisher assigns a DOI when your article is published and made available electronically.

All DOI numbers begin with a 10 and contain a prefix and a suffix separated by a slash. The prefix is a unique number of four or more digits assigned to organizations; the suffix is assigned by the publisher and was designed to be flexible with publisher identification standards.

From: APA Style

Also, some of the article databases such as Ebscohost or ProQuest will include the DOI number within the citation record for an article. The DOI link in the citation record might take you to the publisher's page for the article, not the database article permalink used to to access the article through the library. The publisher's page may put the article behind a paywall once you get there. For that reason, it's good practice to save the database's article permalink in case you need to get back to the result later. (When saving the article URL, be sure to get the "share" link or the "permalink" instead of simply copying the link in the address bar. That link is only good for the current browsing session.) However, some citations styles do not require a DOI to be included in a citation. 

When placing an interlibrary loan request, you do not need to include it unless it is an article lacking other identifying information. 

Fore more information about DOI, see the International DOI Foundation's website.