100 - Continue: This is an intermediate message indicating to the client that the server is working on the request but it may take a few seconds to respond. This status does not often appear in the web log. 
200 - Success: This is the most common status and indicates that the request was successful. 
301 - Moved Permanently: The content exists at a new location permanently. The reference to the item should be changed. This status doesn't occur often in a Windows IIS world, but is used by some other web servers. 
302 - Moved Temporarily: The content exists at a new location, but only temporarily. The client should continue to use the same reference to the content without changing. This is common when Response.Redirect is used in ASP or ASP.NET. 
304 - Not Modified: The browser requested the content only if it had changed since a date provided by the client. This response indicates to the browser that the content hadn't changed and won't be retransmitted. This status indicates that the browser is working to prevent the retransmission of data - which is a good thing. 
401 - Unauthorized: The page requires authorization the client does not possess. This can be caused by a lack of authentication with a specific user name or can indicate that the user doesn't possess authorization to the requested resource. 
403 - Forbidden: Most generally, this error is seen when the web server is set to require HTTPS/SSL for access to a specific URL but the client hasn't requested the URL with HTTPS/SSL. 
404 - Not Found: The page requested wasn't found on the web server; therefore, the request could not be fulfilled. 
500 - Internal Server Error: A problem occurred processing the page, but the error occurred in the internal processing of the request. It doesn't indicate an issue with the browser request; instead, it indicates a problem with the server.