404

404 is an HTTP status code. When you visit a web page, your computer is requesting data from a server using Hypertext Transfer Protocol (HTTP). Before the web page is displayed, your browser sends the web server its HTTP header. The server responds with its HTTP header, which contains the status code. For a normal web page, the status is 200 (OK). You don't see this because the server just sends you the content of the page. If you encounter an error, then you see the actual status code, such as 404 Not Found. The first 4 indicates a client error. The server is saying that you've done something wrong, such as misspell the URL or request a page which is no longer there. (For comparison, a 5xx error indicates a server-side problem, which may be transient; if you try it again, it may work.) The middle 0 refers to a general syntax error. This could indicate a spelling mistake. The last 4 just indicates the specific error in the 40x group, which also includes 400: Bad Request, 401: Unauthorized, etc.