Ten things you need to know about Activity in Salesforce

1. Activity contains Event and Task, they are stored as two separate objects, but you can run report as Activity which contain data from both object.

2. Completed Task will move from Open Activities to Activity History in page layout related list, the same for Event when Due Date is over.

3. By default, there is no Task or Event tab - Activities Tab and View (Event or Task)

4. Activities archive over one year - Archived Activities

5. Not possible to create custom lookup field on Activity (delivered in Winter '16)

6. Cannot assigned to a Queue ; vote for this idea Allow Task(s) to be assigned to a Queue

7. Sharing setting only Controlled By Parent or Private

8. There is no option to create workflow with email alert action - No Email Alert action for Task & Event Workflow ; vote for this idea Email Alerts for Activity Workflows

9. Workflow with field update action on Task cannot update some standard Task fields, like: Due Date, Name, Related To

10. You can create Task from workflow, but it only will have one: Name (Who) or Related To (What), it depends on the Task action related to which object.