Bonus

Definition
Every Cooperative Mission has a Bonus Mission. The bonus is added to your quest log when you aquire it. Bonus mission objectives are obtained either at the start of the mission or while playing through the mission, but it is not guaranteed that your will find the bonus. It's quite possible to finish many missions without knowing what the bonus was, unless you search the mission thoroughly. You get 1000 experience when you complete a bonus, and sometimes 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.