URL: Uniform Resource Locator, a web address

ISBN: International Standard Book Number, a product identifier used by publishers, booksellers, libraries, internet retailers and other supply chain participants for ordering, listing, sales records and stock control purposes. The ISBN identifies the registrant as well as the specific title, edition and format

DOI: Document Object Identifier. This is a unique identifier that is assigned to an online journal article, online book or online book chapter

PMID: unique identifier used in the PubMed database and can be used to look up abstracts in PubMed

arXiv ID: ArXiv is a public server repository that hosts electronic e-prints of research output primarily from the fields of physics, mathematics, computer science, quantitative biology, quantitative finance, statistics, electrical engineering and systems science, and economics