Auto-responders are a tool built into most message management systems. Whether you are using Mail Chimp, AWeber, Constant Contact, IContact or almost any other quality messaging service, an auto-responder will be built into the product suite.
What they allow you to do is send out a series of messages to your e-mail subscriber customer at intervals without manually having to do any work beyond the original setup of your e-mail campaign series.
For example, when they first subscribe to you, it is customary to send them a welcome message that tells them a little more about you or your organization