Above we have looked at just a few of the available HTTP codes; there are many others. Some are very well-known, such as 404 Not Found, but others are quite obscure and don't crop up very often. Whatever the case, the first digit is always an indication of the family of codes they belong to:
1xx (Informational)
: Request received, continuing process.2xx (Successful)
: The action was successfully received, understood, and accepted.3xx (Redirection)
: Further action needs to be taken in order to complete the request.4xx (Client Error)
: The request contains bad syntax or cannot be fulfilled.5xx (Server Error)
: The server failed to fulfil an apparently valid request.
More informations,
http://dev.opera.com/articles/view/http-response-codes/