URIs are similar to URLs in that they specify the location of a file. However, a URI may refer to all or part a URL. For example, Apple's iMac Design URL is http://www.apple.com/imac/design.html. The URI of this resource may be defined as just "design.html" or "/imac/design.html." These are called relative URIs since they identify the resource relative to a specific location. The complete URL would be referred to as an absolute URI.
Because URLs and URIs are similar, they are often used interchangeably. In most cases, this is acceptable since the two terms often refer to the same thing. The difference is that a URI can be used to describe a file's name or location, or both, while a URL specifically defines a resource's location.