The Vault-Tec Assisted Targeting System, or V.A.T.S., is a combat system in Fallout 3 that allows players to pause the game during combat and target specific enemy body parts.

Pausing the game via V.A.T.S. gives the player a chance to catch his breath. Often, the player will spot enemies that went unnoticed in the heat of combat. Each part of an enemy that can be targeted and attacked is highlighted. The general health of each part, as well as the chances of landing a successful hit, is displayed on the screen.Actiontrip: Fallout 3 Interview

Carrying out actions with V.A.T.S. requires Action Points. When the Action Points run out, the game reverts to real-time and the player must wait for the AP to regen.Joystiq: Fallout 3 gameplay details revealed

It should be noted that while using V.A.T.S., the game is not paused completely. The player continues to move in real-time and targeted enemies move at one-tenth their normal speed, while only the surrounding environment pauses fully.Planet Fallout: Fallout 3 VATS

If used strategically, the V.A.T.S. system can give players the upper hand in battle. Gavin Carter, a lead producer at Bethesda, provided an example. Using V.A.T.S., the player might notice that he has a high chance to hit the enemy's torso, but also that one more risky hit to the enemy's arm will severely cripple him. The player's decision can make a huge difference, particularly in long battles.Actiontrip: Fallout 3 Interview

Players are not obligated to use the V.A.T.S. system at all during the game. Fallout 3 can be played as a straightforward real-time first-person shooter.Planet Fallout: Fallout 3 VATS