Activities are the elements that actually do something in a process. There are three types of activities:
Task: The smallest unit of work in a process, for instance the prompt to fill out a form or a call to an external web service.
Call Activity: Executes another, previously defined process which is useful when a model becomes too large or when the same sequence of steps is required in different locations.
Sub-process: Allows to model a process within a process which is useful when working with loops, transactions and compensation.