404: A 404 not found error code is a standard HTTP response code that usually appears when a client was able to communicate successfully with a server, but the server was unable to locate the requested URL because of several possible issues.
Usually, a site hosting server will generate this code when a client requests a webpage through a broken or dead link. This issue is fairly common, so it's one of the most easy-to-recognize online browsing errors.
404 errors may also indicate that the deactivated link may be available again in the future, but the content may not be identical.
404 errors are not to be mistaken for errors like "server not found," which indicates that the client was unable to connect to the host server at all. It also shouldn't be used to signify documents that do exist but can't be accessed. 200 status codes should be used for files that exist instead. In the case of files that have been moved permanently, a 301 code should be used in place of a 404.
Different browsers and websites may customize the appearance and message of their 404 error page to display their brand name, a more natural sounding apology, or even a joke.