Explain the different categories of HTTP status codes and give common examples for each category.

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HTTP status codes are organized into five categories, identified by their first digit.

1xx codes are informational. They indicate that the request was received and processing continues. Code 100 Continue means the server is waiting for the rest of the request. These codes are rarely used directly in APIs.

2xx codes indicate success. Code 200 OK is the standard response for a successful request. Code 201 Created indicates a resource was created successfully, typically after a POST. Code 204 No Content means success but no content to return, often used after a DELETE.

3xx codes concern redirections. Code 301 Moved Permanently indicates the resource has been permanently moved. Code 304 Not Modified means the resource has not changed since the last request, useful for caching.

4xx codes signal client-side errors. Code 400 Bad Request indicates a malformed request. Code 401 Unauthorized means authentication is required. Code 403 Forbidden indicates access is denied even with authentication. Code 404 Not Found means the resource does not exist. Code 422 Unprocessable Entity indicates syntactically valid but semantically incorrect data.

5xx codes signal server-side errors. Code 500 Internal Server Error is a generic server error. Code 502 Bad Gateway indicates a problem with an intermediate server. Code 503 Service Unavailable means the service is temporarily unavailable.

Using the right status codes makes the API clearer and facilitates debugging.