A curse, or hex, is the willful direction of negative magic towards another person. Typically a curse or hex will develop slowly and gradually, increasing the individual's suffering over time. A Pagan or Wiccan who is adept at shielding and magical self-defense is far less likely to become a victim of a curse or hex.
Many magical traditions forbid cursing or hexing, but in some, it is perfectly acceptable. If you're not sure if this applies to you, be sure to read about magical ethics.
Willow refused to direct a curse at the person who insulted her, because she was concerned about retribution.