DOI stands for Digital Object Identifier and is used (in the academic setting) to uniquely identify journal articles. While hosting locations and URLs can change, a DOI remains fixed making it easier to locate in the future. A DOI is typically located on the first page of an article and can also be found in the database citation information (see attached graphic). Older articles or certain publishers may not have a DOI assigned to their articles.

Also, if one has a DOI, it is possible to retrieve the article or object attached to it by looking it up at DOI.org or LibKey