What are activity entites in MS CRM

Activities are tasks that you or your teams perform when they contact customers, for example, sending letters or making telephone calls. You can create activities for yourselves, can assign them to someone else, or can share them with other users or teams. An activity is any action which can be entered on a calendar and has time dimensions (start time, stop time, due date, and duration) that help determine when the action occurred or is to occur. 

As per business requirements we can create our own custom activity entity. And, we can create 1:N relationship with any other entities as just like normal entity.

Below are the list of features available for  activity entity.

1. It has default offline availability.
2. Mail merge is disabled for activity entity.
3. Activity entity ownership can be either team-owned or user-owned. You cannot set it as organization level.
4. You cannot display activity entity in sales, marketing or any other areas.
5. By default Feedback, Notes,  Connections and Queues are enabled by default we cannot change them.
6. Knowledge management cannot be enabled.
7. By default primary field for activity entity is subject which is a single line of text with 200 max length. We cannot change it.
8. A custom activity entity must not have relationships with activities.