An H-drive drivetrain is a system used for heavy off-road vehicles with 6×6 or 8×8 drive to supply power to each wheel station.
H-drives do not use axles but rather individual wheel stations, usually carried on a punt chassis. A single differential splits the drive into separate left and right drive shafts, which each run fore and aft inside the bottom corners of the chassis. At each wheel station a bevel box drives the half shaft out to the wheel.
Six-wheel drive (6WD or 6×6) is an all-wheel drive drivetrain configuration of three axles with at least two wheels on each axle capable of being driven simultaneously by the vehicle's engine. Unlike four-wheel drive drivetrains, the configuration is largely confined to heavy-duty off-road and military vehicles, such as all-terrain vehicles, armored vehicles, and prime movers.
When such a vehicle only has six wheels by definition all are driven. When it has ten—with two pairs of ganged "dual" wheels on each rear axle as on a GMC CCKW—all are also driven but the 6×6 designation remains. For most military applications where traction and mobility are considered more important than payload capability, single wheels on each axle (often referred to as super singles) are the norm.
View the full Wikipedia page for 6×6