DOI is a string of numbers and sometimes letters that uniquely identifies an article and locates it online. This makes it very easy to locate the article.
Where do you look for the DOI?
Here is a DOI found in a citation of an article found in Academic Search Premier [the tutorial calls this the "database landing page"]. The DOI is located on the second line in this example.
An online tutorial provides the history of this identifier, how to find it, and where to cite when creating a reference in Noodlebib.
The International Standard Serial Number (ISSN) identifies specific journals.
Example from journal article found in Academic Search Premier and the ISSN is located at the bottom of the citation.
Here is an example of a book found in the online catalog. The ISBN is listed under the Publisher information. Books found on websites such as Amazon, list this type of information as well.