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In computing, a Uniform Resource Identifier (URI) is a compact string of characters used to identify or name a resource on the Internet.

A URI may be classified as a locator (URL) or a name (URN), or both.
A Uniform Resource Name (URN) is like a person’s name, while a Uniform Resource Locator (URL) is like his or her street address. The URN defines an item’s identity, while the URL provides a method for finding it.

A typical URN is the ISBN system for uniquely identifying books. ISBN 0486275574 (urn:isbn:0-486-27557-4) cites unambiguously a specific edition of Shakespeare’s play "Romeo and Juliet". In order to gain access to this object and read the book, one would need its location: a URL address. A typical URL for this book is a file path, like file:///C:/MyDocuments/RomeoAndJuliet.pdf, identifying the electronic book saved in a local Windows PC hard disk. So the purposes of URNs and URLs are complementary.

A URN can be used to talk about a resource without implying its location or how to access it.

The HTTP specification uses the more general concept of URIs as its resource identifiers; in practice, however, HTTP applications deal only with the URL subset of URIs.

Status Code:

The textual phrase is included only for descriptive purposes; the numeric code is used for all processing.

The following status codes and reason phrases are treated identically by HTTP software:

200 OK

200 Document attached

200 Success

200 All's cool, dude

General URL components(and URL encoding applied on not only http but also mailto, ftp and so on):

<scheme>://<user>:<password>@<host>:<port>/<path>;<params>?<query>#<frag>

Params: the path component for HTTP URLs can be broken into path segments. Each segment can have its own params. For example:

http://www.joes-hardware.com/hammers;sale=false/index.html;graphics=true

query: There is no requirement for the format of the query component. By convention, many gateways expect the query string to be formatted as a series of "name=value" pairs, separated by "&" characters.

Frag: A name for a piece or part of the resource. The frag field is not passed to the server when referencing the object; it is used internally by the client. It is separated from the rest of the URL by the "#" character.

URLs are a powerful tool. Their design allows them to name all existing objects and easily encompass new formats. They provide a uniform naming mechanism that can be shared between Internet protocols.