What is a DOI or PubMed Identifier (PMID)? 

DOI stands for Document Object Identifier.  This identifier labels an individual article (like a serial number) in order that a permanent URL can be created. When searching many databases, the article description will often include a DOI:

The first part before the / is the publisher portion of the number. The second part points to the journal and article specifically.

To learn more about DOIs, see: http://www.doi.org

A PubMed Identifier (PMID) is a unique number assigned to each PubMed citation of life sciences and biomedical scientific journal articles: 

PubMed identifiers allow you to search within the PubMed database using the PMID to access the article information. You can also search Entrez: the Life Sciences Search Engine (accessible via our PubMed subscription) using PubMed Identifiers.