HTTP Response Codes

2xx Success
200 OK; the request was fulfilled.
201 OK; following a POST command.
202 OK; accepted for processing, but processing is not completed.
203 OK; partial information--the returned information is only partial.
204 OK; no response--request received but no information exists to send back.
3xx Redirection
301 Moved--the data requested has a new location and the change is permanent.
302 Found--the data requested has a different URL temporarily.
303 Method--under discussion, a suggestion for the client to try another location.
304 Not Modified--the document has not been modified as expected.
4xx Error seems to be in the client
400 Bad request--syntax problem in the request or it could not be satisfied.
401 Unauthorized--the client is not authorized to access data.
402 Payment required--indicates a charging scheme is in effect.
403 Forbidden--access not required even with authorization.
404 Not found--server could not find the given resource.
5xx Error seems to be in the server
500 Internal Error--the server could not fulfill the request because of an unexpected condition.
501 Not implemented--the sever does not support the facility requested.
502 Server overloaded--high load (or servicing) in progress.
503 Gateway timeout--server waited for another service that did not complete in time.

for IIS special and user defined error codes see MS Management-Console

Links (die offiziellen W3-Spezifikationen) (Method-fields)