Official Gmail Blog: New in Labs: Canned Responses
If you have to reply same response to many like if everyone is wishing you happy birthday through email and you have to reply" thanks a lot for wishing me "many number of times then instead of doing it many time just compose once and save the text message with the canned respones button. Latter, simply open the same message and send it again and again.
Gmail lets you now create filters based on combination of keywords, sender , recipients and more in your incoming messages. turn on you C Response , set your filter to grab any one of the saved response , and create and auto reply, and click send button for you