The Internationalized Resource Identifier is defined similarly to a URI, but the character set is extended to the Universal Coded Character Set. URIs are the glue that binds the Web together. IRIs extend and strengthen the glue, by allowing people to identify Web resources in their own language.
While URIs are limited to a subset of the ASCII character set, IRIs may additionally contain most characters from the Universal Character Set (Unicode/ISO 10646), including Chinese, Japanese, Korean, and Cyrillic characters.
In addition, many W3C specifications -such as XML, RDF, XHTML and SVG-needed a definitive reference for identifiers that support international characters. The IRI specification provides that critical reference.
There is a defined surjective mapping of IRIs to URIs: Every IRI can be mapped to exactly one URI, but different IRIs might map to the same URI. Therefore, the conversion back from a URI to an IRI may not produce the original IRI.