Stands for "Uniform Resource Identifier." A URI identifies the name and location of a file or resource in a uniform format. It includes a string of characters for the filename and may also contain the path to the directory of the file. URIs provide a standard way for resources to be accessed by other computers across a network or over the World Wide Web. They are used by software programs such as Web browsers and P2P file-sharing programs to locate and download files.
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.