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).
Washington University in St. Louis can only provide DOIs for faculty, staff, students, and researchers affiliated with the university.
For other DOI providers, visit DataCite -- please note members have different policies on assigning DOIs.