What is the difference between a URI, a URL, and a URN?

URIs identify and URLs locate; however, locators are also identifiers, so every URL is also a URI, but there are URIs which are not URLs.

Examples

• Roger Pate

This is my name, which is an identifier. It is like a URI, but cannot be a URL, as it tells you nothing about my location or how to contact me. In this case it also happens to identify at least 5 other people in the USA alone.

• 4914 West Bay Street, Nassau, Bahamas

This is a locator, which is an identifier for that physical location. It is like both a URL and URI (since all URLs are URIs), and also identifies me indirectly as "resident of..". In this case it uniquely identifies me, but that would change if I get a roommate.

I say "like" because these examples do not follow the required syntax.

Popular confusion

From Wikipedia:

In computing, a Uniform Resource Locator (URL) is a subset of the Uniform Resource Identifier (URI) that specifies where an identified resource is available and the mechanism for retrieving it. In popular usage and in many technical documents and verbal discussions it is often incorrectly used as a synonym for URI, ... [emphasis mine]

Because of this common confusion, many products and documentation incorrectly use one term instead of the other, assign their own distinction, or use them synonymously.

URNs

My name, Roger Pate, could be like a URN (Uniform Resource Name), except those are much more regulated and intended to be unique across both space and time.

Because I currently share this name with other people, it's not globally unique and would not be appropriate as a URN. However, even if no other family used this name, I'm named after my paternal grandfather, so it still wouldn't be unique across time. And even if that wasn't the case, the possibility of naming my descendants after me make this unsuitable as a URN.

URNs are different from URLs in this rigid uniqueness constraint, even though they both share the syntax of URIs.

From RFC 3986:

A URI can be further classified as a locator, a name, or both. The term "Uniform Resource Locator" (URL) refers to the subset of URIs that, in addition to identifying a resource, provide a means of locating the resource by describing its primary access mechanism (e.g., its network "location"). The term "Uniform Resource Name" (URN) has been used historically to refer to both URIs under the "urn" scheme [RFC2141], which are required to remain globally unique and persistent even when the resource ceases to exist or becomes unavailable, and to any other URI with the properties of a name.

So all URLs are URIs, and all URNs are URIs - but URNs and URLs are different, so you can't say that all URIs are URLs.

If you haven't already read Roger Pate's answer, I'd advise doing so as well.

In summary: a URI identifies, a URL identifies and locates.

Consider a specific edition of Shakespeare's play Romeo and Juliet, of which you have a digital copy on your home network.

You could identify the text as urn:isbn:0-486-27557-4.
That would be a URI, but more specifically a URN* because it names the text.

You could also identify the text as file://hostname/sharename/RomeoAndJuliet.pdf.
That would also be a URI, but more specifically a URL because it locates the text.

*Uniform Resource Name

(Note that my example is adapted from Wikipedia)

These are some very well-written but long-winded answers. Here is the difference as far as CodeIgniter is concerned:

URL - http://example.com/some/page.html

URI - /some/page.html

Put simply, URL is the full way to indentify any resource anywhere and can have different protocols like FTP, HTTP, SCP, etc.

URI is a resource on the current domain, so it needs less information to be found.

In every instance that CodeIgniter uses the word URL or URI this is the difference they are talking about, though in the grand-scheme of the web, it is not 100% correct.

URI => http://en.wikipedia.org/wiki/Uniform_Resource_Identifier

URL's are a subset of URI's (which also contain URNs).

Basically, a URI is a general identifier, where a URL specifies a location and a URN specifies a name.

Another example I like to use when thinking about URIs is the xmlns attribute of an XML document:

<rootElement xmlns:myPrefix="com.mycompany.mynode">
    <myPrefix:aNode>some text</myPrefix:aNode>
</rootElement>

In this case com.mycompany.mynode would be a URI that uniquely identifies the "myPrefix" namespace for all of the elements that use it within my XML document. This is NOT a URL because it is only used to identify, not to locate something per se.

They're the same thing. A URI is a generalization of a URL. Originally, URIs were planned to be divided into URLs (addresses) and URNs (names) but then there was little difference between a URL and URI and http URIs were used as namespaces even though they didn't actually locate any resources.

Due to difficulties to clearly distinguish between URI and URL, as far as I remember W3C does not make a difference any longer between URI and URL (http://www.w3.org/Addressing/).

URI is kind of the super class of URL's and URN's. Wikipedia has a fine article about them with links to the right set of RFCs.

Wikipedia will give all the information you need here. Quoting from http://en.wikipedia.org/wiki/URI:

A URL is a URI that, in addition to identifying a resource, provides means of acting upon or obtaining a representation of the resource by describing its primary access mechanism or network "location".

A URI identifies a resource either by location, or a name, or both. More often than not, most of us use URIs that defines a location to a resource. The fact that a URI can identify a resources by both name and location has lead to a lot of the confusion in my opinion. A URI has two specializations known as URL and URN.

A URL is a specialization of URI that defines the network location of a specific resource. Unlike a URN, the URL defines how the resource can be obtained. We use URLs every day in the form of http://stackoverflow.com, etc. But a URL doesn’t have to be an HTTP URL, it can be ftp://example.com, etc.

Although the terms URI and URL are strictly defined, many use the terms for other things than they are defined for.

Let’s take Apache for example. If http://example.com/foo is requested from an Apache server, you’ll have the following environment variables set:

• REDIRECT_URL: /foo
• REQUEST_URI: /foo

With mod_rewrite enabled, you will also have these variables:

• REDIRECT_SCRIPT_URL: /foo
• REDIRECT_SCRIPT_URI: http://example.com/foo
• SCRIPT_URL: /foo
• SCRIPT_URI: http://example.com/foo

This might be the reason for some of the confusion.

See this document. Specifically,

a URL is a type of URI that identifies a resource via a representation of its primary access mechanism (e.g., its network "location"), rather than by some other attributes it may have.

It's not an extremely clear term, really.

URIs came about from the need to identify resources on the Web, and other Internet resources such as electronic mailboxes in a uniform and coherent way. So, one can introduce a new type of widget: URIs to identify widget resources or use tel: URIs to have web links cause telephone calls to be made when invoked.

Some URIs provide information to locate a resource (such as a DNS host name and a path on that machine), while some are used as pure resource names. The URL is reserved for identifiers that are resource locators, including 'http' URLs such as http://stackoverflow.com, which identifies the web page at the given path on the host. Another example is 'mailto' URLs, such as mailto:fred@mail.org, which identifies the mailbox at the given address.

URNs are URIs that are used as pure resource names rather than locators. For example, the URI: mid:0E4FC272-5C02-11D9-B115-000A95B55BC8@stackoverflow.com is a URN that identifies the email message containing it in its 'Message-Id' field. The URI serves to distinguish that message from any other email message. But it does not itself provide the message's address in any store.

The best (technical) summary imo is this one

IRI, URI, URL, URN and their differences from Jan Martin Keil:

IRI, URI, URL, URN and their differences

Everybody dealing with the Semantic Web repeatedly comes across the terms IRI, URI, URL and URN. Nevertheless, I frequently observe that there is some confusion about their exact meaning. And, of course, others noticed that as well (see e.g. RFC3305 or search on Google). To be honest, I even was confused myself at the outset. But actually the issue is not that complex. Let’s have a look on the definitions of the mentioned terms to see what the differences are:

URI

A Uniform Resource Identifier is a compact sequence of characters that identifies an abstract or physical resource. The set of characters is limited to US-ASCII excluding some reserved characters. Characters outside the set of allowed characters can be represented using Percent-Encoding. A URI can be used as a locator, a name, or both. If a URI is a locator, it describes a resource’s primary access mechanism. If a URI is a name, it identifies a resource by giving it a unique name. The exact specifications of syntax and semantics of a URI depend on the used Scheme that is defined by the characters before the first colon. [RFC3986]

URN

A Uniform Resource Name is a URI in the scheme urn intended to serve as persistent, location-independent, resource identifier. Historically, the term also referred to any URI. [RFC3986] A URN consists of a Namespace Identifier (NID) and a Namespace Specific String (NSS): urn:: The syntax and semantics of the NSS is specific specific for each NID. Beside the registered NIDs, there exist several more NIDs, that did not go through the official registration process. [RFC2141]

URL

A Uniform Resource Locator is a URI that, in addition to identifying a resource, provides a means of locating the resource by describing its primary access mechanism [RFC3986]. As there is no exact definition of URL by means of a set of Schemes, "URL is a useful but informal concept", usually referring to a subset of URIs that do not contain URNs [RFC3305].

IRI

An Internationalized Resource Identifier is defined similarly to a URI, but the character set is extended to the Universal Coded Character Set. Therefore, it can contain any Latin and non Latin characters except the reserved characters. Instead of extending the definition of URI, the term IRI was introduced to allow for a clear distinction and avoid incompatibilities. IRIs are meant to replace URIs in identifying resources in situations where the Universal Coded Character Set is supported. By definition, every URI is an IRI. Furthermore, 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. [RFC3987]

Summarizing we can say:

IRI is a superset of URI (IRI ⊃ URI)
URI is a superset of URL (URI ⊃ URL)
URI is a superset of URN (URI ⊃ URN)
URL and URN are disjoint (URL ∩ URN = ∅)

Conclusions for Semantic Web Issues

RDF explicitly allows to use IRIs to name entities [RFC3987]. This means that we can use almost every character in entity names. On the other hand, we often have to deal with early state software. Thus, it is not unlikely to run into problems using non ASCII characters. Therefore, I suggest to avoid non URI names for entities and recommend to use http URIs [LINKED-DATA]. To put it briefly: only use URLs to name your entities. Of course, we can refer to existing entities named by a URN. However, we should avoid to newly create this kind of identifiers.