Response Codes are defined as part of the HTTP/1.1 protocol (RFC 2068; See Chapter 10). These codes are generated by the web server and indicate the completion status of each request made to it. Some common codes are:

Code 200 - OK
Code 206 - Partial Content
Code 301 - Moved Permanently
Code 302 - Found
Code 304 - Not Modified
Code 400 - Bad Request
Code 401 - Unauthorized
Code 403 - Forbidden
Code 404 - Not Found
Code 408 - Request Timeout