An autoresponder is basically a pre-written email or series of emails that goes out by either you subscribing someone to it manually or by them subscribing themselves. Prospects and customers can subscribe themselves by filling out an online opt-in form. You can set up certain pre-written emails to go out on a delay. So for example, automatically after requesting info from your website, the prospect receives info immediately but then also receives a follow up email a day or two later.
There are no limits on how many autoresponder campaigns you can set up. And each autoresponder campaign can contain as many messages as you'd like!