Error pages are the pages displayed when a user tries to access a location on your site and encountered an error. There are many different types of errors, the most common ones are:
301 - The page has been moved Permanently
401 - The user does not have permission to view the file/directory.
404 - The page could not be found (bad URL).
500 - One of your scripts is not functioning properly, and is returning an Internal Server Error.
Error pages can be modified from the server default by adding custom error handling in your .htaccess file.