It can be very useful to have a standard email response to use for responding to types of emails that you receive often.  Gmail Canned Responses allows you to save a standard message and insert it in an email response whenever you like.

Note that Canned Responses is an option in Gmail Labs and they disclose, “Gmail Labs is a testing ground for experimental features that aren’t quite ready for primetime. They may change, break or disappear at any time.”

I recommend that when you are creating a canned response, you create it in a brand new email that you are composing.  If you create it in an email to which you are replying, the canned response will include the sender’s message too!  (Can you tell I learned this from personal experience?)