Universal Resource Identifier (URI)
Universal Resource Identifier is a unique sequence of characters that identifies a logical or physical resource used by web technologies.
URI are strings, with a special format, also called scheme, which allows identify uniquely a resource, without providing its location.
For example, a ISBN is a unique universal identifier for a book, yet it doesn’t mention where to find such book.
URI is closely related to URL, which provides the location of the desired resource.