To be a little more specific, Pu.ly automatically monitors Twitter (about once every couple of minutes) for mentions of you, direct messages to you, and names of Twitter lists you are added to.

When any of these monitored actions occur, puly immediately sends you an email with the details. You can then simply click the reply button from your email client and send a response that gets published back to Twitter.

Once you’ve set up your pu.ly account, you can also send email to twitter at pu.ly any time and the body of your email will pass directly through to Twitter.