An Autoresponder, is for sending out a series of emails at a specific interval, in order, starting at the time the subscriber signed up. A scheduled email is a single email you send out to your list on a specific date, in the future.
So, let's say you have a 10-part instructional series, and you'd like people to receive one lesson a week. You would pre-write those lessons and set your Autoresponder to go out weekly. Then, no matter when someone signs up, they'll always receive the first lesson in the series, and receive the rest of the lessons once a week from then on. So, if someone signs up in December and the next person signs up in February, they both still get the lessons weekly, starting from the first one.
Looking for more Autoresponder details? Try these links: