Block Unwanted Referers .htaccess / General Guides
Why would you want to block a referer? Well, for one, there are some sites that use webcrawlers that store a url of a page from that site and transmit it as a referering URL when they access your site. They do this thinking (and hopeing) that the URL will go into a publically available hit log for your website and that the URL will be displayed as a link back to their site resulting in their site getting a higher page ranking in search engines.
Customizable 404 pages .htaccess / Error Handling
A 404 error page comes up whenever someone clicks a broken link, or goes to a URL on your site that doesn't exist. To make a custom 404 error page you must first make a page that you want to be your 404 error page. It can have images, CSS, a message about the error, etc.