What is the difference between HTTP 400 and 500 series errors?

HTTP 400 series errors (client errors) indicate that the request sent by the client contains an error or is incorrect. For example, 400 Bad Request indicates incorrect request syntax, 401 Unauthorized means lack of authorization, and 404 Not Found indicates that the resource is missing.

HTTP 500 series errors (server errors) mean that the client's request was correct, but the server could not process it due to an internal error. For example, 500 Internal Server Error is a general server error, 502 Bad Gateway is a gateway error, and 503 Service Unavailable means the server is temporarily unavailable.

Thus, the 400 series are problems on the client side, and the 500 series are problems on the server side.