What is an ISBN, an ISSN, or a DOI?
These three things are different types of unique identifiers.
International Standard Book Number is assigned to books, and book-like products. Such as eBooks, audiobooks, and graphic novels
International Standard Serial Number is assigned to serial publications, or an ongoing resource. Such as periodicals, magazines, journals, and comics.
Digital Object Identifier can be assigned to anything. It identifies an object, both physical and digital, on a large scale or at a granular level. A DOI can apply to an article in an academic journal or painting in an art gallery.