What is 404?
HTTP Error 404 is an error message which means that the web page you are looking for cannot be found.
The 404 error is also known as "File not found", "Not Found" and "Gone".
The Official Definition of 404 Quoting from RFC 2616 - Hypertext Transfer Protocol -- HTTP/1.1
404 Not found
The server has not found anything matching the Request-URI. No indication is given of whether the condition is temporary or permanent. The 410 (Gone) status code SHOULD be used if the server knows, through some internally configurable mechanism, that an old resource is permanently unavailable and has no forwarding address. This status code is commonly used when the server does not wish to reveal exactly why the request has been refused, or when no other response is applicable.
What to Do When You Get a 404 Error
Make sure the URL is entered correctly
Try the URL with a different file extension (.html, .htm, .shtml, .php)
Try deleting the filename from the URL, to see if there is anything viewable in the directory
Go to the main page of the site and look for a search function
Go to Google and search for a similar page
HTTP Error 403
HTTP Error 403 is an error message which means that you are not authorized to view the web page which you are attempting to load.
The 403 error is also known as "Forbidden".
The 403 Forbidden errors is most often seen when you are attempting to browse to a directory which has directory browsing permissions turned off.
The actual error message you receive will depend upon your web browser. For example, Internet Explorer says "You are not authorized to view this page."
The Official Definition of 403 Quoting from RFC 2616 - Hypertext Transfer Protocol -- HTTP/1.1:
The server understood the request, but is refusing to fulfill it. Authorization will not help and the request SHOULD NOT be repeated. If the request method was not HEAD and the server wishes to make public why the request has not been fulfilled, it SHOULD describe the reason for the refusal in the entity. If the server does not wish to make this information available to the client, the status code 404 (Not found) can be used instead.