The default calendar software on the iPhone is ideallly suited to view and edit events from iCal or Outlook which are assigned date and time. Such events are automatically synchronized via iTunes.
Other than these events though, iCal also knows todos which are not scheduled and contain neither time nor date. These todos are also displayed in Apple Mail. (Simple ‘task notes’ which you check of as you handle them. Samples would be “Go get the milk!” or “Call Bob tonight”)