Events/Tasks : Open Activities/Activity History

 There are actually four relationships for activities: 

Tasks, 

Events, 

OpenActivites

ActivityHistories. 

The two real objects are Tasks and Events

OpenActivities show open tasks and events today or in the future.

ActivityHistories shows closed tasks and events before today (in the past). 

When you want to show the history or future activity of a record, you use the two "hybrid" objects

When you want to create a real task or event, you use the two real objects (They'll automatically appear in the appropriate hybrid relationship).