Autoresponders are programs set up to automatically respond with a designated email when a blank email is sent to a certain email address.

Usually it sends a pre-written email message back with information that is setup within seconds.

They "work" 24 hours a day, 365 days per year non-stop unless you set it up to stop. They automate your marketing efforts.

I personally use them for 3 things.

1. In Sales Letters
2. Customer Support
3. Email List Building

There are free ones which contains advertisements in every email sent out and there are paid ones...

Personally i use Aweber. It's a paid service of course.