A 404 error message is the standard HTTP standard response code which is returned when the visitor cannot communicate with the server. Visitor gets a standardized undesirable 404 Error Page
This is a very common error on the web and it occurs when you are trying to visit a page which has either been deleted or has been moved somewhere else.
For example, if you change the structure of your website and move a certain directory to a different part of your site, anyone trying to visit the old page url will get a 404 error message.
404 Error Messages
A 404 error is pretty much lost traffic with in your website. You have managed to get the visitor there with your SEO efforts but a changed page over site results in the visitor getting the generic server 404 error messages. A lot of standard 404 messages are useless and do not even refer your visitor back to the homepage. A 404 error message usually looks something like this :
The requested URL /index.php was not found on this server.
Additionally, a 404 Not Found error was encountered while trying to use an ErrorDocument to handle the request.
Apache/2.2.9 (Unix) mod_ssl/2.2.9 OpenSSL/0.9.7a mod_bwlimited/1.4 PHP/5.2.6 Server at yourwebsite.com Port 80