A Workflow trigger is an event, contact update or contact addition that starts a Workflow. For example, if you create a Workflow that sends a welcome email to new contacts, the trigger would be when a new contact is added to your audience.
There are currently three types of triggers that you can use to start a Workflow: Contact Added to lists,Contact removed from list, Contact Updated, and Event Received.
The Contact added to list trigger will start a Workflow when a contact is added to a mailing list. It triggers every time a contact is added to a list.
The Contact removed from list trigger will start a Workflow when a contact is removed from a mailing list. It triggers every time a contact is removed from a list.
You can change the trigger type at any time. For example, if you create a Workflow with the Contact Added trigger, you can change it to the Contact removed trigger at any time.