What is a DOI?
A DOI is a Digital Object Identifier. It is a permanent alphanumberic code that serves as a unique identifier for an online document. Think of it as an id number for a journal article. It is meant to be more stable than a URL. Most times the DOI of an article (if there is one) is listed on the document's first page. Some citation styles, such as APA, want you to include the DOI when available.