Briefly, Auto Responder (Auto Email Responder) is a program that sends automatic reply in response to an email to a specific email address. However nowadays, the term Auto Responder is generally being used for Sequential Auto Responders.

A sequential Auto Responder is a program that is designed to collect email addresses and then send out a series of customized messages through emails to the list of subscribers on a predefined intervals. These days auto responders are widely used in Internet Marketing. They maintain electronic mailing lists and send emails to specific subscribers at regular intervals. Shortly it's like running your own Ezine/Journal with lot of subscribers.

If your auto responder needs are simple, such as merely sending a single message reassuring your customers that you will get back to them, you can probably use the free auto responders that usually come as part of your web hosting package. The procedure to set up an auto responder varies from web host to web host, but it involves lot of technical knowledge.

If you need an auto responder to send more than one message, or if you need to schedule messages to be sent at certain intervals over a period of time, you will either need to program your mail system on your domain, or use one of the commercial auto responder systems around.

However,for purposes like Email Marketing,(where you promote products using your mailing lists) I always recommend you to Sign up for a commercial auto responder. In commercial auto responders, you can create customized and targeted messages. Also, many of the free auto responders post advertisements in their emails. Moreover,you can maintain unlimited number of email lists with commercial Auto responders.