4WD, 4x4 ("four by four"), are terms used to describe a four-wheeled
vehicle with a drivetrain that allows all four wheels to receive power from
the engine simultaneously. While many people think exclusively of off-road vehicles,
powering all four wheels provides better control on slick ice and is an important
part of rally racing on mostly-paved roads.
Four-wheel drive (4WD or 4x4 for short) was the original term, often used to
describe truck-like vehicles that required the driver to manually switch between
a two wheel drive mode for streets and a four-wheel drive mode for low traction
conditions such as ice, mud, or loose gravel.