Each Digital Object Identifier has a unique alphanumeric string consisting of two parts:
The prefix is a numeric string beginning with 10. and followed by several more numbers, usually four (ex. 10.1103). The prefix is assigned by a Registration Agency to a publisher, institution, organization, or other types of Registrant.
The local name (suffix) follows the forward slash and is a unique alphanumeric string (for that prefix). The local name for an object is chosen by the Registrant using whatever naming scheme they want (ex. PhysRevLett.116.061102).
For this example, the DOI would be:
A DOI handle can be converted into a useful URL by adding it as a path after the domain https://doi.org.
So, for the DOI example above, the full linkable URL would be:
Note that older DOI URLs use http://dx.doi.org which was deprecated in 2016 (but is still supported).