A digital object identifier (DOI) is a unique identifier associated with an object, such as an electronic document. A publisher might assign a journal article a DOI when it is published online; however, DOIs may be assigned to more than just articles. [Other examples of objects which sometimes have DOIs include images, data and data sets, books, and book chapters.] The benefit of a DOI is it is a persistent identifier that is available and managed over time; this means it will not change if the item or object is moved or renamed (From EZID).
A DOI is a character string (a "digital identifier") used to uniquely identify an object such as an electronic document. Metadata about the object is stored in association with the DOI name and this metadata may include a location, such as a URL, where the object can be found. The DOI for a document remains fixed over the lifetime of the document, whereas its location and other metadata may change. Referring to an online document by its DOI provides more stable linking than simply referring to it by its URL, because if its URL changes, the publisher need only update the metadata for the DOI to link to the new URL. From Wikipedia:Digital Object Identifier - Creative Commons Attribution-ShareAlike License