1xx INFORMATIONAL
Request was received; processing can continue.
101 Switching Protocols - Tells the client that the server will switch protocols to that specified in the Upgrade message header field during the current connection.
2xx SUCCESS
The request was successfully received, understood, and accepted.
200 OK - The request sent by the client was successful.
202 Accepted - The request has been accepted for processing, but has not yet been processed.
204 No Content - The request was successful but does not require the return of an entity-body.
3xx HTTP REDIRECTION
Further action must be taken in order to complete the request.
301 Moved Permanently - The resource has permanently moved to a different URI.
302 Found - The requested resource has been found under a different URI but the client should continue to use the original URI.
303 See Other - The requested response is at a different URI and should be accessed using a GET command at the given URI.
304 Not Modified - The resource has not been modified since the last request.
4xx CLIENT ERROR
The request contained bad syntax or cannot be fulfilled.
400 Bad Request - The syntax of the request was not understood by the server.
401 Not Authorised - The request needs user authentication.
403 Forbidden - The server has refused to fulfill the request.
404 Not Found - The document/file requested by the client was not found.
405 Method Not Allowed - The method specified in the Request-Line is not allowed for the specified resource.
407 Proxy Authentication Required - The request first requires authentication with the proxy.
5xx SERVER ERROR
The server failed to fulfill an apparently valid request.
500 Internal Server Error - The request was unsuccessful due to an unexpected condition encountered by the server.
502 Bad Gateway - The server received an invalid response from the upstream server while trying to fulfill the request.
503 Service Unavailable - The request was unsuccessful due to the server being down or overloaded.
504 Gateway Timeout - The upstream server failed to send a request in the time allowed by the server.
Note
These codes are taken from the HTTP/1.1 protocol. A full listing of codes is in Internet RFC 2626, available at the IETF. More detailed comments on the codes can be found in Wikipedia.