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URL - (Uniform Resource Locator) is the address that defines the route to a file on the Internet.

URI (Uniform Resource Identifier) - A more general term for which. URL was a specific type of URI. It is now also used to define a scheme which is the first part of a URL.

Most web addresses (URLs) are of the form http://www.w3.org/Protocols/Specs.html
Which says to use the http scheme (protocol) to go to the http server on the www host at the w3.org domain in the Protocols path (folder/sub-directory) and read the Specs.html file. The general syntax as defined in RFC 1738 is: <scheme>:<scheme-specific-part>
Some scheme-specific-parts are:
IP to host on the Internet: //<user>:<password>@<host>.<domain>:<port>/<url-path>
local machine: "///path" or "/filename"
mail or netnews: "email address" or "newsgroup"
The scheme is defining the type of service the browser will use to access on the server. The most common type is http. URI Schemes are registered at IANA (Internet Assigned Numbers Authority)

user:password@ is an otpional login used for a few services like telnet.

The domain is defining the Internet domain name like w3.org.

The host is defining the domain host. If omitted, the default host for http is www.

The :port is defining the port number at the host. The port number is normally omitted. The default port number for http is 80. 

The path is defining a path (a sub directory) at the server. If the path is omitted, the resource (the document) must be located at the root directory of the Web site.

The filename is defining the name of a document or program. The default filename might be default.asp, or index.html or something else depending on the settings of the Web server. See file type below.

Http is the most common scheme used on the web, but Browsers can also access servers using other (schemes). Following are some examples of the most common types. Some such as ftp, file and mailto work automatically and others require some configuration.

Note: Common types http, https, ftp, file are built into browsers.
Others like mailto, and news are built into Netscape, but with Internet Explorer (IE) will default to Outlook Express for mail and news. 

Scheme        Description and Example
______________________________________
http:   Hypertext Transfer Protocol - File on a World Wide Web server.
     e.g. http://www.w3.org/Protocols/
https:  Hypertext Transfer Protocol Secure - 
        http over SSL (Secure Socket Layer) for encryption. 
ftp:    File on a File Transfer Protocol Server.
        e.g. ftp://ftp.research.telcordia.com/ 
file:   This is similar to ftp but usually denotes file on a local host
         that is not necessairly available over the internet.
         e.g. file:///C|My_Documents/ on Windows
        
mailto: Send mail. (Note: you may need to specify a mail program in your browser)
        e.g. ()
news:   Read news on a Usenet newsgroup.
    e.g. news:comp.infosystems.www.browsers.misc
  (Note: You may need to specify a netnews reader program in your browser.)

Others:
telnet:  Open an interactive terminal window with a telnet server.
Most of the following require configuration to specify an external application.
imap: Internet Message Access Protocol (mail access)
ldap: Lightweight Directory Access Protocol
nntp: USENET news using NNTP (Net News Trsnsport Protocol) access
pop:  Post Office Protocol v3 (mail access)
rtsp: Real Time Streaming Protocol (Was PNM 

im: Instant Messaging - part of Instant Messaging and Presence Protocol (IMPP)
pres: Presence - part of Instant Messaging and Presence Protocol (IMPP)
sip: session initiation protocol -
   For instant messaging and netphone (VoIP).
h323:  ITU-T Standard H.323 - multimedia communications services over packet based networks
       RFC3508 used for NetMeeting to CUworld or Intel NetPhone
daap:  Digital Audio Access Protocol (Apple)
itms: iTunes Music Store (Apple Music downloads)
netphone: Voice over IP (VoIP) phone calls
tel:  Telephone e.g. tel:1-212-555-1212
fax: FAX
modem: Modem

Older schemes not popular now:
gopher: A gopher server. (Similar to ftp but with file descriptions)
wais:   A wide area information server. e.g. WAIS directory-of-servers index
File Type
In addition to the scheme which tells the browser how to access the file, the File Type determined by the "File Name Extension" will determine hot to display it when it arrives. The browser can display some files directly (e.g. .html, .txt, .gif, .jpg) while others require plug-ins (e.g. .pdf) or links to other programs (e.g. .doc [MS Word], .xls [MS Excel]) or Windows Media Player (WMP) for audio/video or the RealOne Player for streaming audio.

Some scheme names are the same as the protocol they use e.g. ftp and others are different. Some common protocols or signaling for a given scheme are:
Scheme Protocol/Signaling
http HTTP - Hypertext Transfer Protocol
ftp FTP - File Transfer Protocol
mailto SMTP - Simple Mail Transfer Protocol
news NNTP - NetNews Transport Protocol
tel DTMF Dual-tone_multi-frequency signaling
See Also:
RFC 1738 - URL
URI Schemes at W3.org and IANA
HTML URLs
File Name Extension
Protocols and Ports
Internet Scheme Syntax
URL Syntax

last updated 27 Apr 2004
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