Activities are the key stages in the projects or processes you manage. Furthermore, activities are usually completed through the execution of tasks. In the system, tasks belong to activities and an activity could have several tasks. When all activity tasks are executed, so would the activity.
When in doubt, remember:
For every key moment/step in the process, you create an activity
For every action you need to take on an activity, you create a task
Naturally, to complete an activity you need to take action. Sometimes this means creating and assigning tasks to a colleague. In other situations, a PM would own the process and they won’t need to create tasks, but simply perform the needed action, and reflect in the system the outcome.