A URL is a string that specifies the particular document or resource
that a browser wants to access. It comes in several components and
is fiendishly complex in terms of what is and is not permitted.
Generally, it comprises:
A protocal (such as http: )
A domain (such as www.google.com )
A path within the domain
A document at the end of that path
Parameters to be passed to any processing done in the fetch.
The above is crude an inaccurate, but gives the right idea.