What is Trigger-based campaign?

Trigger-based campaigns are event based campaigns that are launched on the occurrence of some specific activity. For instance, you may want a contact to receive your Follow-up email after 5 days the contact opened your first email but has not done the activity that you wanted him to do. Trigger-based campaign allows you to design such campaign that will launch based on the set of Rules that you define in your criteria.
Trigger-based campaigns are mainly useful when you have some pre-designed emails that you want to send whenever the contact does some activity and these campaigns will keep or running continuously. You can use a Trigger-based campaign to send the email containing Log-in details whenever a person registers on your website. You may then want to send an email guiding the customer how to use your application / product / software after 2 days of sending the Log-in email.
These are repetative activities and you want all such emails to go at the right time without manual interpretation.
Trigger campaign will let all these happen automatically.