HttpStatusCodes

Success
200 OK

201 CREATED

202 Accepted

203 Partial Information

204 No Response

File Trouble
400 Bad request

401 Unauthorized

402 PaymentRequired

403 Forbidden

404 Not found

Redirection
301 Moved

302 Found

303 Method

304 Not Modified

Server Trouble
500 Internal Error

501 Not implemented

502 Service temporarily overloaded

503 Gateway timeout 503

Link
the HTTP status codes: http://www.w3.org/Protocols/HTTP/HTRESP.html

also see: Apache