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The 404 or Not Found error message, as commonly encountered when using a web browser on the World Wide Web, is a manifestation of one of the standard response codes of the Web's principal communication protocol, HTTP.
When communicating via HTTP, a server is required to respond to a request, such as a web browser's request for an HTML document (web page), with a numeric response code, sometimes followed by an email-like MIME message. Each response code has an associated string of English text that must also be present. Response code 404's associated string is "Not Found". When sending a 404 response, web servers usually include in the response message a short HTML document that mentions both the numeric code and this string. Many browsers opt to show this message to the user, while some browsers instead display a "friendlier" error message.
A 404 response indicates that the web browser was able to communicate with the server, but the server either could not find what was requested, or was unwilling to fulfill the request for it and did not wish to reveal the reason why.
HTTP response codes have a specific interpretation. In code 404, the first "4" indicates a client error such as a mistyped URL. The middle 0 refers to a general syntax error. The last "4" just indicates the specific error in the group of 40x, which in this case is "URL Not Found". The use of three-digit codes is similar to the use of such codes in earlier protocols such as FTP and NNTP.
These errors can be customized on a large number of such servers to display a page that could be of more help than a default. For example, this can be achieved in Apache by placing a .htaccess file on the Web server. Internet Explorer will not read these pages, however, unless they are larger than 512 bytes. Creating humorous 404 pages has become popular and some websites have been created that link to numerous amusing 404 error pages.
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