You can set triggers on Mail lists and on Campaigns.
Trigger on Maillist
When clicking on the edit button next to a maillist, you can set a trigger. Here you can set a trigger when a subscriber joints your maillist, or of if a subscriber unsubscribes from that list. Choose first the action, and than the Mail Campaign. This is the email you created which will be sent to your subscriber or un-subscriber.
Trigger on Campaign
Same as on the Maillist, only here you can choose 4 different triggers. When your subscriber Openend, Not Opened, Clicked or Not Clicked the previous email. First you set your action, than the email which needs to be sent. You can whether let the system sent it immediately