All-Terrain Vehicles (ATVs) are small vehicles with three or more low-pressure tires designed for traveling on rough terrain and a seat that is straddled by the driver. They typically do not have room for passengers, although some "tandem ATVs" have been developed that allow for one passenger in addition to the driver.
It is illegal to drive an all-terrain vehicle on any public street in California or most other U.S. states. These vehicles are meant to be driven on specially designed courses. Most motocross courses are designed to accommodate ATVs.
It is illegal to damage public property by driving ATVs off of established trails where not explicitly permitted. The deep treads on ATV tires that help enable these vehicles to travel on rough terrain are also prone to causing severe ecological damage when these vehicles are driven outside of established trails.