In short, Activities are things planned for guests and Tasks are things assigned to staff.
Activities include things like transfers, tours, spa treatments, concierge referrals, and equipment rentals. Activities will show up on the guest itinerary.
Tasks would include special housekeeping and maintenance tasks, front desk & reservations tasks, general reminders, phone messages, etc. The guest does not directly see these tasks.
All Activities and Tasks are set up initially in the Resources section of Planniac.