PMID (PubMed ID) is the unique identifier for a publication's abstract in PubMed.

PMCID (PubMed Central ID) is the unique identifier for a free complete article (once it is publicly available) in PubMed Central.

PubMed and PubMed Central, produced by the U.S. National Library of Medicine, are text archives of biomedical and life sciences literature.

A DOI (Digital Object Identifier) is a publisher’s unique identifier for a publication. The prefix http://dx.doi.org/ is presumed. Publishers may charge for access.