HTTP Status Code
From Lunarpages Web Hosting Wiki
Below is a list of the more common HTTP Status Codes.
401 Unauthorized: Similar to a 403 Forbidden error, but specifically for use when authentication is possible but has failed or not yet been provided. The response must include a WWW-Authenticate header field containing a challenge applicable to the requested resource.
403 Forbidden: The request was valid but the server is refusing to respond to it. Unlike a 401 Unauthorized response, authenticating will make no difference.
404 Not Found: The requested resource could not be found but may be available again in the future. Subsequent requests by the client are permissible.
500 Internal Server Error: A generic error message, given when no more specific message is suitable. See also: 500 Error and PHP.
500 Resource Unavailable: The server either does not recognize the request method, or it lacks the ability to fulfill the request. A 500 Not Implemented or 500 Resource Unavailable occurs when a script or process attempts to go above the allowed resources for the account.