DOI (Digital Object Identifier) is a standardized string to uniquely identify an item, generally an electronic document. Its most common use is for identifying academic papers that are available online (perhaps free, perhaps paywalled). They are issued by a nonprofit organization, the International DOI Foundation, and can be resolved (if the publisher has registered an official URL for the document) through the servers of that organization. If the document moves, the publisher can update the URL so the DOI continues to resolve.
The format of DOIs is standardized as ANSI/NISO Z39.84-2005. It is usually a string of numbers, dots, and slashes, though any Unicode characters may occur in the portion after the slash (which is assigned by the publisher). Before the slash is a dotted pair of numbers where the first number indicates the registry and the second number the registrant.
info: URI scheme defined in RFC 4452,
info:doi: is a prefix that can be placed before a DOI to make it a valid URI, though browsers don't generally resolve it directly. The CNRI Handle Extension for Firefox will resolve URIs that use a
doi: scheme preceding a DOI, which is used directly as a URI scheme. The DOI organization has chosen not to register DOI as a URN namespace, though it would likely qualify.
A DOI can be resolved by preceding it with
There is also an open-access resolver which uses free versions of papers where available: