Using canned responses
1. Click the gear icon in the upper right-hand corner of Gmail, then choose Settings.
2. Click the Labs tab, find Canned Responses, click the Enable radio button to enable Canned Responses, scroll down and click Save Changes.
3. Compose an email message you’d like to use over and over again, and then click the arrow in the lower right-hand corner of the message window. Choose Canned Responses, and under the Save heading, select New Canned Response. Give your response a name and click OK.
4. The next time someone emails you something that warrants the canned response you created, reply to the message and instead of typing your response, click the arrow button in the lower right-hand corner, select Canned Responses, and then under Insert, choose the canned response you created in the previous step. You’ll then see your response appear in the body of your reply.
5. You can also automate canned responses to be sent out based on certain criteria (sender, keyword, label, subject and things like that)
If you're sick of typing out the same reply every time someone emails you with a common question, now you can compose your reply once and save the message text with the "Canned responses" button. Later, you can open that same message and send it again and again.
It couldn't get any easier unless Gmail automatically pushed the Send button. If you're lazy enough to think that would be a good idea, then read on, friend.Gmail already lets you create filters based on a combination of keywords, sender, recipients, and more in your incoming messages. Turn on Canned Responses in Labs, and you can set a filter to grab one of your saved responses, create an automated reply, and hit the Send button for you.
You can set up different automated messages for different keywords, just like you said you wanted. (We're friends, so I trust you to use this power responsibly.)