HTTP response status codes are issued by a server in response to a client’s request made to the server. (most used)
|Standard response for successful HTTP requests.
|The request has been fulfilled, resulting in the creation of a new resource.
|The request has been accepted for processing, but the processing has not been completed. The request might or might not be eventually acted upon, and may be disallowed when processing occurs.
|204 No Content
|The server successfully processed the request, and is not returning any content.
Client error responses
|400 Bad Request
|The server cannot or will not process the request due to an apparent client error (e.g., malformed request syntax, size too large, invalid request message framing, or deceptive request routing).
|Similar to 403 Forbidden, but specifically for use when authentication is required and has failed or has not yet been provided. The response must include a WWW-Authenticate header field containing a challenge applicable to the requested resource.
|The request contained valid data and was understood by the server, but the server is refusing action.
|404 Not Found
|The requested resource could not be found but may be available in the future. Subsequent requests by the client are permissible.
|405 Method Not Allowed
|A request method is not supported for the requested resource; for example, a GET request on a form that requires data to be presented via POST, or a PUT request on a read-only resource.
Server error responses
|500 Internal Server Error
|A generic error message, given when an unexpected condition was encountered and no more specific message is suitable.
|501 Not Implemented
|The server either does not recognize the request method, or it lacks the ability to fulfil the request. Usually this implies future availability (e.g., a new feature of a web-service API).
|502 Bad Gateway
|The server was acting as a gateway or proxy and received an invalid response from the upstream server.
|503 Service Unavailable
|The server cannot handle the request (because it is overloaded or down for maintenance). Generally, this is a temporary state.
Wikipedia: Status Code