The Meaning of URL, URI, URN and IRI

Some Camel methods take a parameter that is a URI string. Many people
know that a URI is "something like a URL" but do not properly understand the
relationship between URI and URL, or indeed its relationship with other
acronyms such as IRI and URN.
Most people are familiar with URLs (uniform resource locators), such as
"http://...", "ftp://...", "mailto:...". Put simply, a URL specifies the location of a
resource.
A URI (uniform resource identifier) is a URL or a URN. So, to fully understand
what URI means, you need to first understand what is a URN.
URN is an acronym for uniform resource name. There are may "unique
identifier" schemes in the world, for example, ISBNs (globally unique for
books), social security numbers (unique within a country), customer numbers
(unique within a company's customers database) and telephone numbers.
Each "unique identifier" scheme has its own notation. A URN is a wrapper for
different "unique identifier" schemes. The syntax of a URN is "urn::". A URN uniquely identifies a resource, such as a
book, person or piece of equipment. By itself, a URN does not specify the
location of the resource. Instead, it is assumed that a registry provides a
mapping from a resource's URN to its location. The URN specification does
not state what form a registry takes, but it might be a database, a server
application, a wall chart or anything else that is convenient. Some
hypothetical examples of URNs are "urn:employee:08765245",
"urn:customer:uk:3458:hul8" and "urn:foo:0000-0000-9E59-0000-5E-2". The
("employee", "customer" and "foo" in these examples) part
CHAPTER 3 - GETTING STARTED WITH APACHE CAMEL 11of a URN implicitly defines how to parse and interpret the that follows it. An arbitrary URN is meaningless unless: (1) you
know the semantics implied by the , and (2) you have
access to the registry appropriate for the . A registry does
not have to be public or globally accessible. For example,
"urn:employee:08765245" might be meaningful only within a specific
company.
To date, URNs are not (yet) as popular as URLs. For this reason, URI is widely
misused as a synonym for URL.
IRI is an acronym for internationalized resource identifier. An IRI is simply an
internationalized version of a URI. In particular, a URI can contain letters and
digits in the US-ASCII character set, while a IRI can contain those same
letters and digits, and also European accented characters, Greek letters,
Chinese ideograms and so on.


http://camel.apache.org/manual/camel-manual-2.10.0.pdf