Whenever you change something fundamentally on your website, or move it, or something similar, you face the risk of creating outdated links. Links that where posted anywhere else, but the associated page does not exist anymore. 404 Screenshot
Whenever that happens, and you have not taken care of it, the visitor sees a very stripped down page, simply telling him something like "404 Not Found The requested url was not found on this server. file not found". This is neither nice nor useful.

If you want to give your visitors a bit more info and help, it's quite easy to create a customized 404 page.
If your site is running on an apache server, simply put a .htaccess file in your domain's root directory. If your site is running on a windows server, do yourself a favor and move your domain to an apache driven server.

In said .htaccess file you simply put the following text.

ErrorDocument 404 /custom404.htm

Now also create that custom404.htm and fill it with the content you want and style as any other page.
Here you can visit mine.

404 on frank-fuchs.net screenshot