The specification of the URI syntax does not imply anything about
the properties of names and addresses in the various name spaces
which are mapped onto the set of URI strings. The properties
follow from the specifications of the protocols and the associated
usage conventions for each scheme.
URL:
URIs which refer to objects accessed with existing protocols are known as "Uniform Resource Locators" (URLs)
URI is just an abstract syntax to write down 'object reference' in universal way:
scheme:path
path not necessary starts with '//'
and 'scheme' is not necessary an existing protocol
URI, but not URL:
ssn:567-72-5098
Here is more explanation about URLs:
Specific Schemes
The mapping for URIs onto some existing standard and experimental
protocols is outlined in the BNF syntax definition. Notes on
particular protocols follow. These URIs are frequently referred to
as URLs, though the exact definition of the term URL is still under
discussion (March 1993). The schemes covered are:
http Hypertext Transfer Protocol (examples)
ftp File Transfer protocol
gopher Gopher protocol
mailto Electronic mail address
news Usenet news
telnet, rlogin and tn3270 Reference to interactive sessions
wais Wide Area Information Servers
file Local file access
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I refer to tools.ietf.org/html/rfc1630
page 2
URI:
URL:
URI is just an abstract syntax to write down 'object reference' in universal way:
scheme:path
path not necessary starts with '//'
and 'scheme' is not necessary an existing protocol
URI, but not URL:
Here is more explanation about URLs: