There are many dungeons located throughout the world where you will be able to fight powerful monsters for great rewards. The entrance to dungeons can be identified by a swirling portal. Because the dungeons are challenging, you will need to bring five friends with you to defeat the monsters within.
Each dungeon is "instanced" so that each party enters their own private version of the dungeon. This means that you will only be able to see your party members in the dungeon, and you won't be able to see members of another party that are in the same dungeon at the same time.
Dungeons have a wealth of quests, loot, and unique monsters. Each dungeon typically has three or more bosses that drop weapons or pieces of armor that are better than other items of the same level.