Definitions

Event/Trigger

An event happens and triggers something. That something is a push action of data.
Examples: booking is created, booking is changed, customer is created, customer is changed.

Actions

The action is done as a result of a trigger which is fired.
Example: a booking is created. The booking is send to an application which holds a calendar.
The action is to create a calendar item in the target application.

Source application

The application where data is created or changed.
This is usually the application pushing the data via the webhook.

Target application

The application receiving the data from the source application.