These links offer advice for using APA Style and have enough information for most student papers.
The Chicago Manual of Style - 17th ed. - Online
Chicago Manual of Style 16th ed. - OnlineDOI stands for Digital Object Identifier. DOIs are used to identify electronic articles and function much like the ISBNs that you are used to for textbooks.
Unlike a web address, the DOI for an article never changes. APA Style recommends that you include a DOI when available.
Where do I find a DOI?
The DOI is often printed on the first page of journal articles--sometimes at the top of the page and sometimes at the bottom. You will recognize it, because it usually is preceded by doi: and the number always starts with 10.
Some articles will not have a DOI, especially non-scholarly articles or older articles.