An autoresponder is a computer program that generates a response to an email, request, or action. Autoresponders were initially used to notify a sender of an undeliverable email, but now they are often used in email marketing. Autoresponders can give you the ability to send a customer a pre-written email when a specified event takes place, such as a customer creating an account or purchasing a product.
Autoresponders can be outsourced to an Application Service Provider that provides the infrastructure and support for the autoresponder.
Autoresponders can also be installed server-side, giving you complete control while requiring you to provide the infrastructure and technical skill to maintain the autoresponder.
More information about autoresponders can be found here.