DOI stands for digital object identifier. It is a numeric tag that is registered with Cross-Ref or another DOI registrar that is designed to provide a permanent link to the online source of the document or other resource. This ensures that you can still get to a document even though the original source has changed their website or otherwise "moved" the document from its original location. DOIs have two parts -- a prefix that indicates the publisher or source of the material, and a unique alphanumeric identifier for documents from that publisher. Most (but not all) SPE papers have DOI links.