Vehicles can be described by the number of axles. Most light vehicles only have 2 axles. Most vehicles have 4 wheels but only 2 driving wheels.
Some have 4 driving wheels, and these are commonly called 4-wheel-drive vehicles.
On commercial vehicles, the load carried on a single axle is limited by law, so vehicles with extra axles are common. This layout has 6 wheels to support the vehicle but only 2 drive it. The extra axle at the rear is only used to support the weight of the vehicle. This extra axle is sometimes called a lazy axle. This is called a 6-by-2 vehicle.
If the lazy axle is changed to a driving axle, this becomes a 6-by-4 vehicle.
Some heavy goods vehicles have an extra steering axle, which allows more weight to be carried.