A Hypertext Transfer Protocol (HTTP) 404 error, also commonly known as the 404 Page Not Found error, is a response code indicating that the client was able to reach the desired server but could not find the page or file for which it was looking. In simpler terms, the Internet browser was able to connect to the website in question, but the requested page was not found.
Every time a request is made from a client to a server, the HTTP protocol sends what is called HTTP headers with a status code. For a normal website, the status code is "200 OK," which is the standard response for successful HTTP requests. Other status codes might be displayed if the website is redirected.
Each numeral in an HTTP 404 error means something. The first numeral "4" indicates that a client error has occurred, so the server is saying that there was an error on the client's end, such as a typo. It also indicates that the error might not be permanent. The middle "0" indicates a general syntax error. The last "4" indicates which particular error was made out of the 400 client error group, which includes "400 Bad Request," "401 Unauthorized" and "403 Forbidden."
HTTP 404 errors are handled on the server level and not by software running on the website. Some websites will emulate a HTTP 404 error, in which case it is called a soft 404 error. These websites will use a redirection technique such as a meta redirect or will use the .htaccess file to achieve this. The advantages for webmasters of doing this is that they can display whatever message or page they want when the requested page is not found on the server, thus improving the browsing experience.
The HTTP 404 page can be specified in the server configuration, thus providing an actual HTTP 404 error while being able to redirect the user to a page within the website. Using redirects as HTTP 404 errors can be harmful to the website's search engine results pages (SERPs). In some browsers, if a custom HTTP 404 page is below 514 kilobytes in size, the browser will display its own 404 error page and not the one specified in the server configurations.