401 : Unauthorized
Access to the URL resource requires user authentication which either has not yet been provided, or which has been provided but failed. This is commonly known as password protection.
403 : Forbidden
The request was a legal request, but the server is refusing to respond to it. Unlike a 401 Unauthorized response, authenticating will make no difference. This is usually due to a scripting or file permissions issue.
404 : Not Found
This response code indicates that the visitor was able to communicate with the server, but either the server could not find what was requested, or it was configured not to fulfill the request and not reveal the reason. Error 404 should not be confused with "server not found" or similar errors, in which a connection to the destination server cannot be made at all.
500 : Internal Server Error
Your web server encountered an unexpected condition that prevented it from fulfilling the request by the visitor. Basically, something has gone wrong, but the server can not be more specific about the error condition in its response to the visitor.
503 : Service Unavailable
This error is likely due to an issue on your server which will require additional assistance. Typically this can occur when your server is temporarily overloaded or undergoing maintenance