The .htaccess file is a pretty powerful tool, it allows you to password protect folders, redirect users automatically, use custom error pages, change your file extensions, ban users by IP address, only allow users with certain IP addresses, stop directory listings and use an alternate index file.
Creating the file is actually super easy. All you have to do is put in the appropriate code into your favorite text editor, its really not a lot. just remember that when you are saving it, it must read .htaccess (this file is actually nameless, with an 8 letter file extension). Once you have uploaded the file you can then rename it using your FTP program.
this is how easy it is to tell it what to do if an error page is reached:
ErrorDocument
this is how easy it is to tell it what to do if an error page is reached: