Curses

Curses is a Necromancer secondary attribute meaning that any character with a Necromancer for their primary or secondary class can put points into this attribute.

Guild Wars Description
"No inherent effect. Many Necromancer skills, especially hex spells which reduce your foes' effectiveness in battle, become more effective with higher Curses."

General Description
Curses can be described as a support attribute, meaning that while curses skills may not be able to kill an enemy by themselves, they sure make the killing a lot easier. This is because curses are all about weakening your foe in a wide variety of ways.

One way to weaken an enemy is by removing their enchantments, and this is where curses especially shine. With no fewer then three different skills geared towards removing enchantments and punishing the user, those pesky Monks will be much reduced in their effectiveness. A good strategy would be to use Desecrate Enchantments to deal some damage to the user, then either Chilblains or Rend Enchantments to get rid of them.

Curses are also good at handicapping warriors. Using the combination of Price of Failure (foes miss with attacks and take damage when they do), Enfeeble (gives enemy weakness), and Faintheartedness (causes attacks to be 50% slower) makes it very unlikely that Warriors will be able to deal any damage at all.

Curses even have skills to handicap spell-casters. Using both Malaise and Wither will give your target a total of 4 energy degeneration, effectively stopping them from regaining mana. Using Spinal Shivers and one of the many Necromancer skills that cause cold damage (Deathly Chill would be a good choice because of its fast casting) will prevent a mage from casting spells.

Unfortunately, curses are no good on their own. To create an effective curse-user, skills must be placed in other attributes as well (try using the Mesmer attributes Domination Magic or Illusion Magic for hexes galore). Also, due to the low damage curses deal, you'll find that without support from other classes or using other attributes' skills, you can't kill anything with the ability to heal itself.