A digital object identifier (DOI) is a unique alphanumeric string that identifies content and provide a persistent link to its location on the Internet. The publisher assigns a DOI when the article is published and made available electronically.
The DOI system was implemented in 2000 so content published since 2000 is more likely to have a DOI and, in general, journal articles are more likely to have a DOI than other types of material including books.