URI
An URI identifies a resource. It is a locator. It includes a URI scheme, authority, path, query and fragment by syntax. For example, http: is a URI scheme.
A URI identifies a resource either by location, or a name, or both. More often than not, most of us use URIs that defines a location to a resource. The fact that a URI can identify a resources by both name and location has lead to a lot of the confusion in my opionion. A URI has two specializations known as URL and URN.
URN
The term “Uniform Resource Name” (URN) is used to identify a resource independent of its location. Example urn:ISBN:1-23-432536-5
A URI identifies a resource by name in a given namespace but not define how the resource maybe obtained. This type of URI is called a URN. You may see URNs used in XML Schema documents to define a namespace, usually using a syntax such as:
<xsd:schema xmlns="http://www.w3.org/2001/XMLSchema"
xmlns:xsd="http://www.w3.org/2001/XMLSchema"
targetNamespace="urn:example"
Here the targetNamespace use a URN. It defines an identifier to the namespace, but it does not define a location.
URL
The term “Uniform Resource Locator” (URL) refers to the subset of URIs that, in addition to identifying a resource, provide a means of locating the resource by describing its primary access mechanism (e.g., its network “location”).
A URL is a specialization of URI that defines the network location of a specific resource. Unlike a URN, the URL defines how the resource can be obtained. We use URLs every day in the form of http://learnjavaindetail.blogspot.com, etc. But a URL doesn’t have to be an HTTP URL, it can be ftp://learnjavaindetail.blogspot.com, smb://learnjavaindetail.blogspot.com, etc.
