Due Date and Due Time - Setting Alarms
You can set these when you create or edit a task. Here are the possibilities:
1) If you set both the date and the time, a one time alarm will be created. When it goes off, a reminder will appear in the notification bar, the phone will vibrate and will emit some beeps.

2) If you just set the time, a periodic alarm will be created. It will run at the selected time daily or weekly until the task is checked or deleted. As a suggestion, you can make a special tag for these periodic tasks and you can also name it suggestively, like 'Periodic', 'Daily' or 'Weekly'.

3) If you just set the date, a simple notification (without sounds or vibration) will appear on the due date when you start the app.