The DOI (Digital Object Identifier) website will tell you all you want to know, but here's a quick summary, in a Library context:
What?
It's a system for identifying content objects in the digital environment. DOI names are assigned to any entity for use on digital networks. They are used to provide current information, including where they (or information about them) can be found on the Internet. Information about a digital object may change over time, including where to find it, but its DOI name will not change
Where?
Usually found on the first page of an e-journal article, near the author's address or the copyright notice; in the top-left hand corner of abstracts; in the Table of Contents; at the top or bottom of an article, or in the DOI field in the record for an article in a database
DOI's usually appear in the following form:
Digital Object Identifier: 10.4225/01/4F3DB08617645
(The prefix always begins with 10.xxxx/ where x is the unique number given to a publisher in the DOI system, and the suffix indicates the individual item or document).
To convert a DOI into a web link, simply add it to the end of a 'DOI resolver', such as : http://dx.doi.org/
Why?
Using DOI names as identifiers makes managing intellectual property in a networked environment much easier and more convenient, and allows the construction of automated services and transactions
