The NIH recently announced a pilot program to help speed up drug development. Most drug development focuses on finding new molecules or compounds to treat existing disorders. The new program is looking for researchers to find uses for existing compounds, hopefully reducing both the cost and time involved in developing new drugs.
Straight from the wiki: A digital object identifier (DOI) is a character string (a "digital identifier") used to uniquely identify an object such as an electronic document.
It is similar to an ISBN, which serves to uniquely identify books, but uniquely identifies electronic documents. It must be purchased, registered, and maintained. Thus, any document with a DOI is more likely to come from a reputable source.