Task and Email Notification Difference

Hi Team,

I've a business logic to send notification 7 days before and on the day when contract gets expired which I'm trying to implement in both email and task. However, I do not understand the difference between tasks and email notification in D365/CE. 

I've created a workflow to send email and create task on the same when contract expires. But I would also need to do this 7 days before.

Could you please help me to implement both conditions and purpose of task and email.

Thank you,

Leo