404

404 is the server response code for "Document Not Found".

You can do some interesting things with your server to provide a better (or amusing) experience to the user.

Here's a simple example just redirects a person to your homepage for any request that would otherwise be 'not found'. It's not actually all that helpful.

ErrorDocument 404 /

Here's a more interesting example for using an ErrorDocument in a virtual hosting scenario where you want to handle not just missing documents, but also 'missing' domains. The challenge: If there's no DocumentRoot, where will the script live? Alias to the rescue.

<VirtualHost *>
  UseCanonicalName Off
  VirtualDocumentRoot /var/www/clients/%-2/%0/
  ServerName catchall.host
  Alias /errors /var/www/default/errors/
  ErrorDocument 404 /errors/notfound.php
</VirtualHost>

Now, when an unconfigured domain is requested, the script at /var/www/default/errors/notfound.php is invoked instead. This script can check $_SERVER['HTTP_HOST'] to see what domain was requested. If it is actually configured, then we have a regular 404. If it's not configured, we can display some alternate error message. You could redirect to a "default" domain, or show a sign-up screen for your hosting service.