Bonus

Definition
Every Cooperative Mission has a Bonus Mission. The bonus is added to your quest log when you aquire it (During the mission or at the start of it) Yoy are rewarded 1000 experience when you complete a bonus. You might also receive additional rewards.

Tips
A lower level character receives more experience from kills, so you will gain less experience from monsters if you use the bonuses to achieve new levels. You could maximise the benefit of the extra experience by completing the mission bonuses after reaching level 20.