CrossRef is an official Digital Object Identifier (or DOI) Registration Agency. It was launched in early 2000 as a cooperative effort among publishers to enable cross-publisher citation linking in online academic journals. To date, CrossRef is the most robust implementation of the DOI model. It now interlinks millions of items from a variety of content types, including journals, books, working papers, technical reports, and datasets. A digital object identifier (or DOI) is a standard for persistently identifying a piece of intellectual property on a digital network and associating it with related data, the metadata, in a structured extensible way. ... Electronic journals are scholarly journals or magazines that can be accessed via electronic transmission. ...
The expense is paid for by the journal publishers. Initially, there was some resistance from publishers who did not want to link to articles published by some other publisher, but by now every major academic publisher uses the system to their mutual advantage. It is also pssible for users to search for articles directly from the CrossRef website.