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you change something fundamentally on your website, or move it, or something similar, you face the risk of creating outdated links.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.

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. But keep in mind, visitors coming here, did not intend to do so. For them something is broken and they might even think it's their fault. So try to be helpful and show them a way to continue.

You can visit my 404 page intentionally.

404 on frank-fuchs.net screenshot