In Dungeons & Dragons' third edition Player's Handbook, Two-Weapon Fighting is described as a feat (special ability) that your player character (or "PC") can acquire. From the Player's Handbook (p.86):

"You can fight with a weapon in each hand. You can make one extra attack each round with the second weapon.

• Benefit: Your penalties for fighting with two weapons are reduced by 2.
• Normal: See Attacking with Two Weapons, page 124, and Table 8-2: Two Weapon Fighting Penalties, page 125.
• Special: The Ambidexterity feat reduces the attack penalty for the second weapon by 4.
  A ranger wearing light armor or no armor can fight with two weapons as if he had the feats Ambidexterity and Two-Weapon Fighting."

One of the better feats for PCs who want to kick ass, although it is almost worthless without Ambidexterity -- the penalties for two-weapon fighting are typically so high that it's almost not worth it.