It's illegal to drive a car while "impaired" by the effects of alcohol or drugs (including prescription drugs). This means that there must be enough alcohol or drugs in the driver's body to prevent him from thinking clearly or driving safely. Many people reach this level well before they'd be considered "drunk" or "stoned."
Additionally, an adult who has a blood alcohol content (BAC) level of .08% or above is guilty of a DUI (driving under the influence) or DWI (driving while intoxicated) -- no matter how sober the driver feels, or how well that person was driving.
However, almost all states consider drivers under the age of 21 to be driving under the influence of alcohol if their BAC is at or greater than .01% or .02%, depending on the state.