Following is the list of Errors Most People Encountered with these days:
301: Moved permanently means that url will point to another url for always.
302: Moved temporarily means that url will point another for time being.
400: Bad request means due to bad syntax request can’t be fulfilled.
401: Unauthorised means it happened when an authentication required but can’t be fulfilled due to some reason.
403: Forbidden means request made by client is true but server is not responding on it. It doesn't happen because of authentication like in 401.
404: Page not found means page is not available for which client requested for.
500: Internal server error means server have nothing to response for request made by client.
501: Not implemented means either can’t fulfill the request or didn't understand the particular request.
502: Bad gateway means Server is acting like a gateway and receive invalid response.
503: Service unavailable means server is down or overloaded.504: Gateway timeout meane Server acting like a gateway and didn't get timely response from upsteam server.