Auto-replies are one-off messages sent in response to an email or web form submission. They can be sent to anyone, and don’t require that a recipient is assigned to a certain group of contacts, or even has a contact record in your database. Auto-replies are triggered by Filters.
Autoresponder Campaigns are a series of automatic emails sent to a contact at specified intervals. Each autoresponder campaign is associated with a group of contacts and is triggered when a contact is added to the group.