In plain language
200s mean success, 400s mean the request had a problem, and 500s mean the server had a problem.
Example
A 401 means the credentials are missing or invalid; a 429 means too many requests were sent too quickly.
Related terms
HTTP Request · Rate Limit · Retry and Backoff · Error Handling
Common misconception
A 200 response does not always mean the work succeeded; some APIs return 200 with an error inside the body.
Implementation guidance
Branch on status classes, retry only temporary errors such as 429 and 5xx, fix 4xx errors instead of retrying them, and check response bodies for errors.