Digital Object Identifiers (DOI) are the most widely known and used PIDs.
DOIs are persistent unique identifiers designed for research objects, such as articles, books and book chapters, conference proceedings, data sets, etc.  The DOI system is designed to identify objects wherever they are located on the web, unlike a URL which points to a specific location on the web which may change or disappear over time. DOIs alleviate the problem of dead links or link rot. 
DOIs are typically issued at the time of an object's publication, much like an ISBN or serial number. All DOIs begin with a 10 and contain a prefix and a suffix separated by a slash. The prefix is a unique number of four or more digits assigned to organizations; the suffix is assigned by the publisher and identifies the object.
Who manages DOIs?
What should have a DOI?
Where can you find DOIs in documents?
The examples below show the general location of DOIs in digital repository holdings, in this instance Elsevier's ScienceDirect and NTL's ROSAP repositories.
Can’t find a DOI?
Although DOIs are becoming increasingly popular and often required, not all publications have DOIs or list DOIs within databases.
Why use a DOI?
DOIs are persistent, meaning they have been designed with the ability to provide lasting information on where objects or information about them can be found on the internet.  Although Information about an object may change over time, such as where it can be found on the web, its DOI will never change.
The DOI has become an important piece of publication metadata that guarantees continued access to, at minimum, the metadata for an object, and helps researchers track the use of their articles. The ability to track an article's metrics helps researchers measure the impact of their work and the number of times it is cited, discussed, shared, bookmarked, or otherwise used across the Internet.
It is important to note that NTL’s publications are grey literature and not scholarly publications, since it is publicly available gov. data and is often used without proper citations. As a result, the use of metric tools won’t always provide a full picture of where or how the data is being used. We are currently in the process of preparing to share our own metrics within the repository, but this project has not been fully realized yet.
The below tools are not endorsed by NTL, but offer some options for researchers who want to explore the potential reach of their publications. Additionally, each has its own limits on the information it is able to produced, which are mentioned below.
Ways to track your DOIs?
Beyond ensuring continued access to research, DOIs offer the benefit of being traceable across the web, which allows metrics of an article's success to be tracked over time. In the digital world this can extend beyond traditional metrics, such as article citation count. Alternative metrics or altmetrics aim at capturing success through additional means, including article mentions on Twitter, the number of times an article is publicly bookmarked or saved to a citation manager, etc.  The best way to ensure that altmetrics can be collected for an object is to place it in a repository that already has tools in place to track altmetric data, but there are other services that can help gather altmetrics.