Email auto responders will automatically send a customized auto response (that you compose) to any visitor who emails the address configured with one. More specifically, automated responses are sometimes used to send additional information about your service or product by having a visitor email something like moreinfo@your_domain.com. In most other cases, they are used to send a 'courtesy reply' to anyone who sends a query to your companies' main email address. When visitors email this address, they receive a response such as:

'Thanks for contacting our company! Someone will be returning a response to your question soon. If you require immediate assistance, please call 555-555-5555. Thanks!'

Another use is to inform those who email you that you are on vacation and will reply back when you are available again. Be sure to delete these types of auto responders when you are back from vacation!