A triggered email is an automated follow-up sent when a recipient clicks a link or opens an email. For example, send a follow-up 24 hours after someone opens your new product announcement, or send a "thank you" email immediately when they click your purchase link.
Successful email marketers are moving towards greater personalisation and segmentation—away from blasting one email to their entire list. They send highly relevant content to segments based on user profiles and behaviours.