Completing Missions

The first time you finish a mission, you'll receive a 1,000 XP reward for completing it. Each mission also advances the game's story through a series of short cinematics.

You can attempt a mission multiple times, but there's a limit to the reward you can receive for completing it. Each you time you finish a mission, a shield appears on the screen, covered by one, two, or three swords.
One sword crossing the shield is worth 1,000 XP.
The second sword crossing the shield is worth 2,000 XP.
The third sword crossing the shield is worth 3,000 XP.

You can only receive the reward for each sword once.

In Prophecies, you can earn the second or third sword by completing secondary objectives. They're usually related in some way to your primary mission, but have their own independent goals. In addition to experience, you can sometimes earn gold or items (or both) by completing a secondary objective.

When you form a group with other players, you may want to decide early in the mission whether the group wants to attempt a bonus objective (or if you know the objective, decide before you start). http://www.guildwars.com/gameplay/combat/missionsquests/