Custom error pages are displayed when a user enters a wrong URL, an outdated URL or when the user is not authorized to access a specific directory of your website.
Custom error pages can be created from cPanel.
After logging into cPanel go to Advanced -> Error pages
A nice example of a "404 - Not Found Page" can be seen here,
To customize the error page first choose the domain in the drop-list then select the error page number (400, 401, 403, 404, 500) or the error page name (Bad Request, Authorization required, etc).
Once you create the pages you should make sure your .htaccess file in the public_html directory contains one of the lines below
For custom 404 error page: "ErrorDocument 404 /404.shtml"
For custom 500 error page: "ErrorDocument 500 /500.shtml"
If not you may have to create your own ".htaccess" file
Error Pages consists of the following five types of common error pages,
Error - 400 Bad Request
Error - 401 Authorization Required
Error - 403 Forbidden
Error - 404 Wrong Page
Error - 500 Internal Server Error
When the page loads the following tags can be put into your error page
Referring URL - Displays the url of the web site the visitor was previously viewing
Visitor's IP Address - Displays the IP address of the visitor viewing the error page
Requested URL - Displays the intended url the visitor is trying to view
Server name - Displays the web site's server name
Visitor's browser - Displays the visitor's browser type such as IE, FireFox, Netscape, etc
Redirect Status Code - This displays the type of code in the 300-307 range
Choose the desired tags and press 'Save' button. Your Error Page is ready