Abbrev. for uniform resource indicator. An identifier that uses the syntax defined in RFC 3986. URIs are divided into two parts by a colon. The first part is a label indicating the scheme of the URI, and the second part identifies the resource using the syntax appropriate to that scheme. For example, the URI “tel:+44-020-7…” uses the “tel” scheme, which identifies telephone numbers; therefore, the second part uses the standard syntax for telephone numbers (in this case specifying a number in London, England). Internet URLs are a subset of URIs, with the schemes http, ftp, smtp, etc.
http://tools.ietf.org/html/rfc3986 The URI specification