If you change the URL of an existing page or decide to delete it completely from your server, your visitor will get a 404 or “File Not Found” error when they try to access that page.
How to handle a 404 error in your website?
If you change the URL of an existing page or decide to delete it completely from your server, your visitor will get a 404 or “File Not Found” error when they try to access that page.
A 404 error indicates that the requested resource may be available again in the future.
Here i sharing best info about 404 error. If you come across a 404 page, then it should have an email somewhere on the page. You should consider emailing the webmaster and tell him that a link is broken. He will often then, after reading your email, redirect the traffic to his new page with the same content, if it is due to changing scripts.
I think The 404 error shows up when a page is not found, that is if the sever doesn't find that page.
Here some best info about 404 error. The 404 error shows up when a page is not found, that is if the sever doesnt find that page. It does not exist. If you know that the site exists, the 404 error can appear when Yahoo is working on the server. The problem lasts for 2-3 days, then disappears when the work is done. 404 errors can be frustrating because you don't know exactly what is happening. So you don't know how to fix it.
Error 404 page is the page displayed when someone asks for a page that’s simply not available on your site. As there is no web page to display, the web server sends a page that simply says "404 Page not found".
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The first thing you need to do is to add the following line to a file named .htaccess (note the preceding period). In most instances, no such file will exist, and you can simply create one with a text editor (such as Notepad on Windows). Incidentally, if you are using Notepad on Windows, you will need to enter ".htaccess" (including the opening and closing quotation marks) into the "Save As" dialog box when you create the file, otherwise Notepad will append a ".txt" extension behind your back when it creates the file. And unless you configured Windows to show you the full filename, you won't even realize that it has done so.
more information visit this link : http://www.thesitewizard.com/archive/custom404.shtml
This error is easily shown in a Web browser if try a URL with valid domain name but invalid page.
This error occurs in the final step above when the client receives an HTTP status code that it recognises as '404'.