What is Sequence Campaign?

Sequence Campaigns are a kind of drip campaigns that can be used to automatically send a pre-defined set of emails to contacts after regular intervals of time based on their activities.
For example, Sequence Campaign for Events/Conferences. You can create a Sequence Campaign that will send Email1 on sign-up, Email2 to give instructions for the event once the person signs up, Email3 as a reminder a day before the conference, Email4 after the conference giving out some links for downloads,etc.