What is an ISBN, an ISSN, or a DOI?

These three things are unique identifiers:

ISBN

International Standard Book Number - for books, and book-like resources, eg: eBooks, audiobooks, and graphic novels

ISSN

International Standard Serial Number - for serial publications, or an ongoing resource, such as periodicals, magazines, journals, and comics.

DOI

Digital Object Identifier for an object, both physical and digital, large or small. It can apply to an article in an academic journal, an object in a museum, a recorded speech, or even a painting in an art gallery.