A Digital Object Identifier, or DOI, is a persistent means for identifying content online. They are commonly used with academic articles and allow the document to be found, even if its URL is changed. The DOI of a document does not change, and online databases store metadata, including location, of the document. Use this tag if your query is specific to DOIs.
doi is about Digital Object Identifiers.
DOIs usually occur in bibliographic contexts. Consider tagging your query with hyperref, bibliographies, bibtex or biblatex first. If your question is about DOIs and doesn't naturally fit into one or more of these or other tags, then consider doi.