Professor Hornbuckle from U. of Iowa explains what a DOI is and why it is important.
DOI = Digital Object Identifier
Source: APA Style Guide to Electronic Resources (online ebook)
Here are a couple of resources explaining what DOI is, when and why created.
Flowchart showing steps to using a DOI and where to locate it.
A DOI URL looks like this ----- https://doi.org/10.1007/s10754-013-9127-4
A general DOI number is -- 10.1007/s10754-013-9127-4
Two links that help with verifying DOIs:
SHORT Video Clip explaining What a DOI is.