I find it seriously frustrating when servers return cryptic error pages. If you've ever seen a 403, 404 or 500 message, you know what I mean. Well sort of, maybe... What you might not know is that all these messages, and more are called Server Response Codes. Whenever you contact the server and request a page or execute a CGI script (through your browser), a server will process this request and respond with these codes, that signal certain things:

Range	Meaning
100-199	The request was received and is being processed.
200-299	Request processed successfully.
300-399	Request was not processed successfully and redirection in process.
400-499	There was a problem with the request.
500-599	There was a problem with the server itself when trying processing and carrying out the request.

Some of the more common ones:

401 - Authorization Required
You probably failed to provide the correct access information - usually through HTTP authentification.

403 - Forbidden
You are not allowed to view the contents of a certain directory either because the directory was not marked readable and "executable" or the page you were accessing (the index page when you tried to access the directory) did not exist and the server will not display the contents of the directory instead.

404 - Not Found
You tried to access a page that was not on the server (occasionally interchangeable with 403). Either that, or you typed in the URL incorrectly so the page in question could not be located.

500 - Server Error
You probably tried to access a program or script that executes on the server and there was a "compilation" or some other run-time problem with the script. Also happens sometimes when a request to execute a program on the server times out.