Bonus
From Guild Wars Wiki
This article is about optional mission objectives. For the special command to generate bonus items, see /bonus.Bonus is a term used to describe a secondary objectives in each mission. Completing the bonus is optional, but each bonus grants a reward.
In Prophecies, a bonus is a a single secondary objective or side quest inside a mission. Completing one will grant 1000 XP.
In Factions and Nightfall a bonus still a single side objective, but is broken into two parts for grading: an Expert reward worth 1500 XP, 150 
and a skill point, and a Master reward worth 2000 XP, 200 and another skill point. How well you do determines whether you get no reward, just Expert, or both of them.
Factions bonuses are typically for completing the mission in under a certain amount of time, and normal Nightfall ones are a side objective with a success level that can be measured numerically, but both campaigns use the other format occasionally.
In Prophecies, credit for the bonus objective is received as soon as the objective is completed. In Factions and Nightfall, the mission must be completed in order for the bonus to be counted.
To count towards the Protector or Guardian titles for a campaign, a mission must be completed with all bonuses.
