Though it's illegal in every state to drive while under the influence of drugs or alcohol, states differ on how they define the offense. (States also use different terms, like "driving while intoxicated" (DWI) and "operating while impaired" (OWI).)
However, in all states, the offense basically breaks down into two parts:
As is discussed below, state laws differ on how these two elements are defined.
In most states, a motorist can be convicted of a DUI without actually putting the car in motion. All that's required is that the motorist was "operating" or "in actual physical control" of the vehicle. So, depending on what state you live in, being arrested and convicted of a DUI might be possible for just sleeping in a car while drunk. In just a few states—California being one—the prosecution must prove actual driving to get a DUI conviction.
Because state laws vary, DUI defenses related to the "driving" element vary by jurisdiction.
Generally, prosecutors can prove the "under-the-influence" component of a DUI by showing the driver had a blood alcohol concentration (BAC) of .08% or more or was actually impaired by the drugs or alcohol ingested. Generally, impairment can be from alcohol, illegal drugs, or legal drugs like prescription medications and marijuana (in states where it's legal).
An offense based on an excessive BAC level is called a "per se" DUI. (Some states also have per se drug DUI laws that prohibit driving with a certain amount of drugs in your system.) All states have laws that outlaw driving while impaired by drugs or alcohol. But states differ on how "impaired" is defined.