Every standard HTTP status code, in plain language, including what the 4xx and 5xx errors ask you to do.
Status codes are the web's terse vocabulary: the first digit gives the class (1 information, 2 success, 3 redirect, 4 your-request problem, 5 server problem) and the rest give the specifics. This directory covers every standardized code with a plain-language explanation, written for the moment you meet one in the wild, which is usually a browser error page, an API response, or a log line at an inconvenient hour. The ones with the richest real-world behavior, 403, 404, 429, 500, 502, 503, also have full troubleshooting guides in the Runtime Error Index on this site.
A few pieces of folklore worth correcting in print: 404 is not 'the internet is broken', it is the most specific, honest answer a server gives; 418 (teapot) is a joke standardized in 1998 that real systems occasionally use deliberately; and 451 exists because lawyers needed a status code. APIs add conventions on top, 422 for validation failures is the big one, and platforms invent unofficial codes (Cloudflare's 52x family, for instance) that appear only in their own documentation.
What is the difference between 401 and 403? 401 means 'I do not know who you are, authenticate.' 403 means 'I know (or do not care) who you are, and the answer is no.' Retrying with credentials fixes 401s; 403s need permissions or a different door.
Is 404 bad for SEO? A few are normal and harmless (old links die). Sudden thousands signal a broken migration, that is when to audit redirects. 410 tells crawlers to drop a URL faster than 404 does.
What should I do when I see a 5xx as a visitor? Wait and retry, 500/502/503/504 are the server's problems, not yours. If a specific site shows one for hours, that is their outage, and status pages or social posts usually confirm it.