URN vs. URL vs. URI

Uniform Resource Identifier (URI) : Identify
A compact sequence of characters that identifies an abstract or physical resource. Encompases URN's and URL's (and URC's).

Example: "www.google.com", "http://www.google.com"

Uniform Resource Name (URN) : Uniquely name
Identifies a resource by a unique and persistent name, but doesn't necessarily tell you how to locate it on the internet. 

Example use: 6e8bc430-9c3a-11d9-9669-0800200c9a66

Unified Resource Location (URL) : Locate
Contains information about how to fetch a resource from its location. URL's ALWAYS start with protocol. 
Example use: "http://www.google.com"

So if I want to be 'that guy', when should I use which terms? 
Use URI's for most everything, unless it includes protocol ('http://', 'https://', etc) in which case be 'that guy' and say it's a URL. 

Source
https://danielmiessler.com/study/url-uri/