404 Error
The "404" error is when something is not found on a website. It is most often caused by an incorrect link to a page on the website, or a page that is very old and has changed its URL (common for old cross-site links that were never maintained), or was there but is no longer hosted. This is a server side error and has nothing to do with your internet connection, which will more clearly state that it cannot connect to the internet or something similarly broad. Three digit errors that you see indicate a server issue or response. Naturally, 404 is the most well known of them all, and compared to some fairly benign.
There are five ranges in broad; 100s are 'ongoing connection' and are not displayed normally by browsers, 200s are success messages that also aren't displayed and 300s are redirect codes. 400s have a handful to know; 401 is Bad Request indicating an error in the client or attempted connection, 403 Forbidden for no permission (notably often used for image hotlinking; if coming from Google and seeing this, you may be able to manually refresh to bypass the error) and 410 Gone, a more 'sophisticated' error to say that something was removed and it's gone for good. 500 most regularly occurs when a site is malfunctioning and includes 500 Internal Error (generic), 502 Gateway Timeout (server not responding) + 503 (unavailable or overloaded), 508 for an infinate loop and the less dramatic 511, authentication required.
Further reading:
- w:List of HTTP status codes, a breakdown from Wikipedia