What are persistent identifiers?
Persistent identifiers (PIDs) are long-lasting references to digital objects of various types. An identifier is generally any label used to name an object uniquely. URLs are examples of an identifier, but also serial numbers, and personal names. But PIDs can do more than naming an object uniquely. PIDs were invented to address challenges arising from the expanding internet which is constantly changing and makes it hard to maintain a persistence record of digital objects. Often, the address of a digital object changes over time and the information is not reachable any longer, leaving '404' tombstones.
PIDs are labels that locate, identify and share information about digital objects persistently without 'rotting away', because a PID may be connected to a set of metadata describing an object rather than to the object itself. They allow different platforms to exchange information consistently and unambiguously and thus provide a reliable way to track citations and reuse.
There are multiple PID systems for different digital objects, operated by different service providers, and used by different research disciplines. We most commonly distinguish between two varieties of PIDs: PIDs for objects (publications, data, software etc.) and PIDs for people (e.g. researchers). Some PIDs you may have already encountered are an ISBN number on a book, the DOI of a CERN Yellow Report, the identifier given for preprints on arXiv or the ORCID iD on an INSPIRE author profile.
The European Commission project FREYA developed guides that provide a general overview of different PID types for multiple research artifacts. You can consult these guides here in order to get help for choosing the right PID for your use case. The guides were last updated in May 2020.