A more thorough explanation:

Driving under the influence (DUI) is a criminal offense that occurs when a person operates a vehicle while impaired by alcohol, drugs, or other controlled substances. This impairment can cause dangerous driving and put others on the road at risk.

In some states, DUI and driving while intoxicated/impaired (DWI) are used interchangeably. However, in states that charge both offenses, DUI usually refers to cases of alcohol impairment, while DWI usually refers to cases of impairment by recreational or prescribed drugs.

Law enforcement officers can ask drivers to undergo chemical testing of their breath, blood, or urine whenever the officer suspects the driver of impairment. This suspicion is usually triggered when an officer witnesses dangerous or erratic driving. Drivers can refuse chemical testing, but in most jurisdictions, such a refusal will result in an automatic forfeiture of driving licensure.

Testing and arrests for DUI and DWI can also occur at sobriety checkpoints set up by the police. The Supreme Court upheld sobriety checkpoint programs as constitutional in Michigan Dept. of State Police v. Sitz, holding that 4th Amendment concerns over seizure were outweighed by the degree to which the sobriety checkpoint advanced the public interest in road safety.

Most impaired driving offenses are charged after a breathalyzer or blood test indicates that a driver’s blood-alcohol content (BAC) is above the federal BAC limit of 0.08%. State law may enforce a lower legal BAC limit. Some states may pursue DUI/DWI charges at lower levels for drivers under the legal drinking age. Some states may also allow charges to be brought without chemical testing results, if there is a suspicion of impairment due to erratic or dangerous driving coupled with a failed field sobriety test.

• John was pulled over by a police officer for swerving on the road. The officer suspected that John was driving under the influence and asked him to take a breathalyzer test. John refused, and as a result, his driver's license was automatically forfeited.
• Samantha was stopped at a sobriety checkpoint and was asked to take a breathalyzer test. The test showed that her BAC was above the legal limit, and she was arrested for DUI.
• Mike was driving home from a party and was pulled over for speeding. The officer noticed that Mike's eyes were bloodshot and that he was slurring his words. The officer asked Mike to take a field sobriety test, which he failed. As a result, Mike was charged with DUI even though he refused to take a chemical test.

These examples illustrate how DUI charges can be brought against drivers who are suspected of impairment due to alcohol or drugs. In each case, the driver exhibited signs of impairment, and law enforcement officers took action to ensure the safety of others on the road.