The Uniform Resource Locator (URL) is a technical, Web-related term used in two distinct meanings:
- in popular usage, it is a widespread synonym for Uniform Resource Identifier (URI)—many popular and technical texts will use the term "URL" when referring to URI;
- in strict technical usage, it is a subset of URI specific to identifiers who are primarily locators.
The standard pronunciation of “URL” is as an initialism (“U-R-L”), but some people pronounce “URL” as an acronym (“Earl”).
A popular synonym for "URI"
The idea of a uniform syntax for global identifiers of network-retrievable documents was the core idea of the World Wide Web. In the early times, these identifiers were variously called "document names", "Web addresses" and "Uniform Resource Locators". But these names were misleading, since not all identifiers were locators, and even for those that were it was not the defining characteristics. But, by the time the RFC 1630 formally defined the term URI as a generic term best suited for the concept, the term "URL" gained widespread popularity, which has continued to this day.
URI/URL syntax in brief
Here is a typical URI dissected:
\__/ \_________/ \_/\__________/ \_____________/ \___/
| | | | | |
scheme host port path query fragment
Every URI (and therefore every URL) begins with the scheme name which defines its namespace, purpose and syntax of the remaining part of the URI. Most Web-enabled programs will try to dereference a URI according to the semantics of its scheme and a context-specific heuristics. For example, a Web browser will usually dereference a http://example.org by performing a HTTP request to the host example.org, port 80. Dereferencing URI mailto:email@example.com will usually open a "Compose e-mail" window with the address firstname.lastname@example.org in the "To" field.
URLs as locators
In its current strict technical meaning, a URL is a URI which, “in addition to identifying a resource, [provides] a means of locating the resource by describing its primary access mechanism (e.g., its network ‘location’).”