A DOI, or Digital Object Identifier, is a unique string of letters, numbers, and symbols attached to a file or document so that it can always be found on the internet. Even if the URL to the document changes, the DOI will always remain the same.
You can usually find an article or document's DOI with it's title or within the first page.
Examples of DOIs:
An Analysis of the Pricing of Traits in the U.S. Corn Seed Market is 10.1093/ajae/aaq063
The Characterization of Twenty Sequenced Human Genomes is 10.1371/journal.pgen.1001111