What is a DOI?
The Digital Object Identifier (DOI®) System is for identifying content objects in the digital environment. DOI® names are assigned to any entity for use on digital networks. They are used to provide current information, including where they (or information about them) can be found on the Internet. Information about a digital object may change over time, including where to find it, but its DOI name will not change.
How to do it
Let us examine an example: doi: 10.1130/G32273.1 the “doi:” part identifies that what follows is a DOI but it is not part of the DOI itself. In this case the DOI is: 10.1130/G32273.1
To resolve the DOI as a URL(link) we need to precede it by http://dx.doi.org/ so that in the example above the URL(link) becomes http://dx.doi.org/10.1130/G32273.1
For better formatting you can change the text that appears for the link e.g. doi: 10.1130/G32273.1
Why turn a DOI into a URL (link)?
If you are creating or sending any kind of document electronically (e.g. webpage, pdf, word doc, email) then this is useful for the reader, who can simply click on the link and be taken straight to the page. Most software (e.g. ms word, email clients) automatically make URLs clickable.
If you want to resolve any DOI you can of course go to the site: http://dx.doi.org/ and paste the DOI in.