Crossref is a not-for-profit membership organization for scholarly publishing. Unlike PubMed, it is not discipline-specific. Publishers of electronic scholarly content join as members of Crossref and are assigned a unique Digital Object Identifier (DOI) prefix. For each content item a publisher wishes to register in the system, it creates a unique DOI (incorporating the assigned prefix) and tags it to the article’s metadata and the URL where the article resides. The publisher submits the record to the Crossref metadata database. Crossref then registers each article DOI and URL in a central DOI directory. This allows for permanent inbound links to the publisher’s content, because other publishers, librarians, and affiliates (such as subscription agents and secondary publishers) can retrieve from Crossref the DOIs that link to that content. More details about Crossref can be found here.