These are all unique identifiers that refer to a specific source. Based on these identifiers, you should be able to find the full and correct bibliographical information.

• A DOI or Digital Object Identifier is a code distributed by the International DOI Foundation, or IDF. It can refer to all types of sources, but is usually used to denote an article or e-book.
• An ISBN or International Standard Book Number is a code referring to a monograph. It consists of 10 or 13 digits and is requested by the publisher of the monograph. Do you want to request an ISBN? Go to the research tip ISBN for more information.
• An ISSN is an International Standard Serial Number and refers to a serial publication, like a journal.
• A PMID or PubMed ID is a code used for articles in the database PubMed.