What is the difference between an event element and an action element?

With action elements you determine what will happen, and can use them to control the campaign flow, otherwise referred to as the internal program logic. For example, you can use one of these elements to start sending emails or setting tags.

An event is therefore a user's action. Whenever an external event occurs, an event element will react. For example, a user fills out a form and Quentn receives the data.