A URI, or Uniform Resource Identifier, is a sequence of characters that identifies a logical or physical resource used by web technologies. URIs can be used to identify anything, including real-world objects, concepts, or information resources such as web pages and books. Some URIs provide a means of locating and retrieving information resources on a network, such as the Internet or another private network, and these are called Uniform Resource Locators (URLs) . A URL provides the location of the resource, while a URI identifies the resource by name at the specified location or URL.

The terms URI and URL are often used interchangeably, but they have different meanings. A URI is an identifier of a specific resource, while a URL is a specific type of identifier that identifies a resource and specifies how it can be accessed. In other words, a URI simply identifies the resource, while a URL identifies the web address or location of a unique resource. All URLs are URIs, but not all URIs are URLs.

URIs have a scheme that refers to a specification for assigning identifiers within that scheme. The URI syntax is a federated and extensible naming system, where each schemes specification may further restrict the syntax and semantics of identifiers using that scheme. The URI generic syntax is a superset of the syntax of all URI schemes.

In summary, a URI is a unique sequence of characters that identifies a logical or physical resource used by web technologies, while a URL is a specific type of URI that identifies a resource and specifies how it can be accessed.