In computing, a Uniform Resource Locator (URL) is a type of Uniform Resource Identifier (URI) that specifies where an identified resource is available and the mechanism for retrieving it. In popular usage and in many technical documents and verbal discussions it is often, imprecisely and confusingly, used as a synonym for uniform resource identifier. The confusion in usage stems from historically different interpretations of the semantics of the terms involved. In popular language, a URL is also referred to as a Web address.
In its current strict technical meaning, a URL is a URI that, “in addition to identifying a resource, [provides] a means of locating the resource by describing its primary access mechanism (e.g., its network ‘location’).
Computer scientists may classify a URI as a locator (URL), or a name (URN), or both.
A Uniform Resource Name (URN) is like a person's name, while a Uniform Resource Locator (URL) resembles that person's street address. The URN defines an item's identity, while the URL provides a method for finding it.
The ISBN system for uniquely identifying books provides a typical example of the use of typical URNs. ISBN 0486275574 (urn:isbn:0-486-27557-4) cites unambiguously a specific edition of Shakespeare's play Romeo and Juliet. In order to gain access to this object and read the book, one would need its location: a URL address. A typical URL for this book on a unix-like operating system is a file path, like file:///home/username/RomeoAndJuliet.pdf, identifying the electronic book saved in a local hard disk. So URNs and URLs have complementary purposes.
Technical view
A URL is a URI that, in addition to identifying a resource, provides means of acting upon or obtaining a representation of the resource by describing its primary access mechanism or network "location". For example, the URL http://www.wikipedia.org/ identifies a resource (Wikipedia's home page) and implies that a representation of that resource (such as the home page's current HTML code, as encoded characters) is obtainable via HTTP from a network host named www.wikipedia.org. A Uniform Resource Name (URN) is a URI that identifies a resource by name in a particular namespace. A URN can be used to talk about a resource without implying its location or how to access it. For example, the URN urn:isbn:0-395-36341-1 is a URI that specifies the identifier system, i.e. International Standard Book Number (ISBN), as well as the unique reference within that system and allows one to talk about a book, but doesn't suggest where and how to obtain an actual copy of it.
Technical publications, especially standards produced by the IETF and the W3C, have long deprecated the term URL, as it is rarely necessary to distinguish between URLs and URIs. However, in nontechnical contexts and in software for the World Wide Web, the term URL remains ubiquitous. Additionally, the term web address, which has no formal definition, is often used in nontechnical publications as a synonym for URL or URI, although it generally refers only to "http" and "https" URL schemes.