Class D typically refers to a category of driver's licenses that allow individuals to operate standard passenger vehicles. This classification varies by state or country, but it generally includes cars, vans, and light trucks. To obtain a Class D license, applicants usually need to pass a written test, a vision exam, and a driving skills test. In some contexts, "Class D" can also refer to a type of amplifier used in audio equipment. Class D amplifiers are known for their high efficiency and compact size, making them popular in home audio systems and professional sound setups. They work by rapidly switching the output devices on and off to control the power delivered to the speakers.