Status codes

The following is a list of HTTP response status codes and standard associated phrases, intended to give a short textual description of the status. These status codes are specified by RFC 2616, along with additional, unstandardized status codes sometimes used on Web.

The first digit of the status code specifies one of five classes of response.

1xx Informational
Request received, continuing process.
100: Continue
101: Switching Protocols

2xx Success
The action was successfully received, understood, and accepted.
200: OK
201: Created
202: Accepted
203: Non-Authoritative Information
204: No Content
205: Reset Content
206: Partial Content

3xx Redirection
Further action must be taken in order to complete the request.
300: Multiple Choices
301: Moved Permanently
302: Found
303: See Other
304: Not Modified
305: Use Proxy (Many HTTP clients (such as Mozilla and Internet Explorer) don't correctly handle responses with this status code.)
306: (unused, but reserved)
307: Temporary Redirect

4xx Client Error
The request contains bad syntax or cannot be fulfilled.
400: Bad Request
401: Unauthorized (See basic authentication scheme and digest access authentication.)
402: Payment Required
403: Forbidden
404: Not Found
405: Method Not Allowed
406: Not Acceptable
407: Proxy Authentication Required
408: Request Timeout
409: Conflict
410: Gone
411: Length Required
412: Precondition Failed
413: Request Entity Too Large
414: Request-URI Too Long
415: Unsupported Media Type
416: Requested Range Not Satisfiable
417: Expectation Failed

5xx Server Error
The server failed to fulfill an apparently valid request.
500: Internal Server Error
501: Not Implemented
502: Bad Gateway
503: Service Unavailable
504: Gateway Timeout
505: HTTP Version Not Supported
509: Bandwidth Limit Exceeded (This status code, while used by many servers, is not an official HTTP status code.)

Izvor: Wikipedia
More info: Official W3C HTTP status code specification