There are many persistent identifiers (PIDs) for digital objects such as publications, data, and software. The most commonly seen PID systems for objects in the field of scientific information are the following:

Archival Resource Key (ARK)

ARKs are no formal standard, but all ARK systems follow the same structure (they all start with the 'ark:' for example) and workflows. There is also no central resolver. Organizations can sign up to become Name Assigning Authority Numbers (NAANs) and run their own resolution infrastructure for ARKs. The system is run by the California Digital Library with dozens of NAANs worldwide through a combined ARK/DOI infrastructure EZID.

Example: ark:/13030/tf5p30086k

arXiv Identifier

The document server for preprints arXiv developed their own system of PIDs and changed their identifier scheme in 2007. Until 2007, the arXiv identifier included subject-classification information, but was limited to  999 submissions per month in a single archive. As arXiv gained more popularity, they changed the structure of their identifiers. It allows arXiv to handle now more than 9999 submissions per month.

Example: arXiv:1207.7214

Digital Object Identifier (DOI)

The DOI is the most commonly used PID system for publications, data, and software across disciplines and is also broadly used at CERN. DOIs are built upon the older Handle system and were first introduced in 1998 with funding from the International DOI foundation (IDF). In 2012, the DOI system even became an ISO standard (ISO 26324).

DOIs are assigned by DOI Registration agencies. However, building upon the Handle system, there is a central free worldwide resolving mechanism for DOI names which means that DOIs are self-sufficient and their resolution does not depend on a single agency. For every DOI name, a standard metadata kernel has to be defined and the registration of DOI names at DOI registration agencies such as DataCite and Crossref typically involves a fee, but the resolution of DOIs is free.

Examples: 
CERN Yellow report: 10.23731/CYRM-2019-007
Dataset on CERN Open Data Portal: 10.7483/OPENDATA.CMS.6O84.WLN8
Software on Zenodo: 10.5281/zenodo.821635
Research publication: 10.1016/j.physletb.2012.08.020

See how to get a DOI

Handle

The Handle system is a non-commercial identifier resolution system used since 1995. It is a general-purpose global name service enabling secure name resolution operated by the Corporation for National Research Initiatives (CNRI).

A Handle identifier consists of a prefix which identifies the 'naming authority' in combination with a suffix, which states the 'local name' of an object. The location of the object a Handle refers to and metadata that describe the object are stored separately in the Handle system.  A server associates metadata with a Handle, and returns that metadata when requested by a call to the Handle Service. This technique is used as the foundation of other higher level systems such as DOI. Commercial Handle licences can be obtained by research bodies and institutional repositories to establish local Handle systems, such as the European Persistent Identifier Consortium (EPIC).

Example: hdl:2381/12775

International Standard Book Number (ISBN)

The ISBN is a numeric book identifier which is intended to be unique.

An ISBN is assigned to each separate edition and variation (except reprintings) of a publication. For example, an ebook, a paperback and a hardcover edition of the same book will each have a different ISBN. The ISBN is ten digits long if assigned before 2007, and thirteen digits long if assigned on or after 1 January 2007. For more details, see ISBN on Wikipedia

Example: ISBN 978-92-9083-543-1 (paperback), ISBN 978-92-9083-544-8 (PDF)

See how to get a ISBN

International Standard Serial Number (ISSN)

An ISSN is an eight-digit serial number used to uniquely identify a serial publication, such as a magazine. The ISSN is especially helpful in distinguishing between serials with the same title. ISSNs are used in ordering, cataloging, interlibrary loans, and other practices in connection with serial literature.  For more details, see ISSN on Wikipedia

Example: ISSN 2519-8068 (print), ISSN 2519-8076 (online)

See how to get a ISSN

Uniform Resource Name (URN)

URNs were originally thought to be part of a three-part information architecture for the Internet just like the Uniform Resource Locators (URLs). URNs were introduced in 1994, formalised in 1997 and are now an IETF standard. URN systems are widely used by major national libraries in Europe (just like the CERN Library) as ISBNs for books are part of the URN system.

Generally, there are no licence costs involved for assigning URNs, but a URN registration agency needs to establish an assigning and a resolving infrastructure. However, there is no central governance and no central resolving infrastructure, as the URN ecosystem is very large.

Example: urn:isbn:0451450523

See more Unique identifiers on Wikipedia