The request could not be understood by the server due to malformed syntax. The client SHOULD NOT repeat the request without modifications. Text taken from: http://www.submissionchamber.com/help-guides/error-codes.php

The server understood the request, but is refusing to fulfill it. Authorization will not help and the request SHOULD NOT be repeated. If the request method was not HEAD and the server wishes to make public why the request has not been fulfilled, it SHOULD describe the reason for the refusal in the entity. If the server does not wish to make this information available to the client, the status code 404 (Not Found) can be used instead. Text taken from: http://www.submissionchamber.com/help-guides/error-codes.php

All HTTP based exceptions will derive this class.

The server encountered an unexpected condition which prevented it from fulfilling the request.

The requested resource was not found in the web server.

The request requires user authentication. The response MUST include a WWW-Authenticate header field (section 14.47) containing a challenge applicable to the requested resource. The client MAY repeat the request with a suitable Authorization header field (section 14.8). If the request already included Authorization credentials, then the 401 response indicates that authorization has been refused for those credentials. If the 401 response contains the same challenge as the prior response, and the user agent has already attempted authentication at least once, then the user SHOULD be presented the entity that was given in the response, since that entity might include relevant diagnostic information. HTTP access authentication is explained in rfc2617: http://www.ietf.org/rfc/rfc2617.txt (description is taken from http://www.submissionchamber.com/help-guides/error-codes.php#sec10.4.2)