A digital object identifier (DOI) is a permanent, unique string of characters that identifies specific digital content such as an online article, dataset, image, etc.). Ideally, when a DOI is assigned (by a publisher, repository host, etc.), there is metadata and a URL which is associated with the DOI which makes the DOI 'resolvable' (point to the digital object or to a web resource that gives you more information about the digital object).
All DOI numbers begin with a 10 and contain a prefix and a suffix separated by a slash. The prefix is a unique number assigned to organizations; the suffix is assigned by the the organization.
Examples:
10.1159/000330840
10.1289/ehp.1003206
DOIs can often be used in citations in lieu of urls.
