HTTP status codes

The HTTP status codes below are the most commonly encountered status codes.

Code Description
200 Ok.
The HTTP request to the URL was successful.
301 Moved permanently.
The requested URL has been permanently moved to another location (URL). This is the best status code to use when moving indexed pages to another location.
302 Found.
Originally the response was "Moved Temporarily". Implementation of this code was fragmented, so 303 and 307 were introduced. General speaking it´s best to avoid this status code.
303 See other.
The response to the request can be found under another URI using a GET method. When received in response to a POST (or PUT/DELETE), the client should presume that the server has received the data and should issue a redirect with a separate GET message.
403 Forbidden.
Access to the requested URL was denied.
404 Not found.
The requested URL was not found. You´ll want to fix these errors as soon as possible.
405 Method not allowed.
A request method is not supported for the requested resource. Example: a GET request on a form that requires data to be send through POST.
500 Internal server error.
Likely to occur when there´s a misconfiguration of the destination server or when a generic error has been encountered.
503 Service unavailable.
The server is currently unavailable. This could be due to maintenance, or all resources have been used up. Most of the time this is temporary.

More HTTP status codes can be found on Wikipedia or check Section 6.1.1 "Status Code and Reason Phrase" at W3C.