HTTP status codes

When a server responds to an HTTP request, the first line it
returns is the status line. This line contains a three-digit code and
a short description of the status of the response. There are five
classes of codes, explained in the following list:

1## - Informational. These codes are mainly experimental.

2## - Success. The codes in this class are returned on successfully
filled requests. Status code 200 is used to indicate a successfully
retrieved web page.

3## - Redirection. The status codes in this class are used to show
that additional action must be taken before the request can be
satisfied. For example, code 301 can be used to indicate that a page
has been moved and the browser may be redirected to the new page.

4## - Client Error. These codes are returned when a browser has made a
request that can’t be fulfilled. The dreaded “404–URL not found” is
perhaps the most famous of status codes.

5## - Server Error. This class of status codes is used to indicate a
problem on the server end. Sometimes code 503 is used to indicate a
server currently has too many requests to process.

Knowing the status code classifications can sometimes make debugging
web-page problems less difficult.
Source: Some where in the World Wide Web