404 Page Not Found

                      "Error 404" is common error that comes when requested web page not found. When user click on Outdated or Broken link, type wrong URL in address bar that problem comes.

Some website display custom "404 error" and some text message  "Not Found" default web server error. Then you will think why it calls "404 error"  because it produces by web server. when web page not found, these error code recognized by Search engines.
                                             
A "404 error" is an HTTP code that means the page you were trying to open on a website could not be found on server. Technically, an error 404 is a client side error.
                                             
You can read "404 error" with the help Web scripts and Monitoring tools. It help To maintain Broken Links.