Tractor unit in the context of 6x6

A truck (North American and Australian English) or lorry (British English) is a motor vehicle designed to transport freight, carry specialized payloads, or perform other utilitarian work. Trucks vary greatly in size, power, and configuration, but the vast majority feature body-on-frame construction, with a cabin that is independent of the payload portion of the vehicle. Smaller varieties may be mechanically similar to some automobiles. Commercial trucks can be very large and powerful and may be configured to be mounted with specialized equipment, such as in the case of refuse trucks, fire trucks, concrete mixers, and suction excavators. In American English, a commercial vehicle without a trailer or other articulation is formally a "straight truck" while one designed specifically to pull a trailer is not a truck but a "tractor".

The majority of trucks currently in use are powered by diesel engines, although small- to medium-size trucks with gasoline engines exist in North America. Electrically powered trucks are more popular in China and Europe than elsewhere. In the European Union, vehicles with a gross combination mass of up to 3.5 t (3.4 long tons; 3.9 short tons) are defined as light commercial vehicles, and those over as large goods vehicles.

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.

A ballast tractor is a specially weighted tractor unit of a heavy hauler combination. It is designed to utilize a drawbar to pull or push heavy or exceptionally large trailer loads which are loaded in a hydraulic modular trailer. When feasible, lowboy-style semi-trailers are used to minimize the height of a load's center of mass. Typical drivetrains are 6×4 and 6×6, but 8×6 and 8×8 are also available. Typical ballast tractor loads include oil rig modules, bridge sections, buildings, ship sections, and industrial machinery such as generators and turbines.

Only a handful of manufacturers produce dedicated ballast tractors. Extra-heavy-duty chassis versions of mass-production tractor units are fitted with drawbar hitches and a separate ballast box as an alternative. These units are classified as N3 Category of large goods vehicle. Ballast tractors can be traced back to the 1940s when heavy haulers from the UK started employing purpose-built Scammell Showtracs, a short wheelbase 4×2 ballast tractor.

An artillery tractor, also referred to as a gun tractor, is a specialized heavy-duty form of tractor unit used to tow artillery pieces of varying weights and calibres. It may be wheeled, tracked, or half-tracked.

A semi-trailer truck (also known by a wide variety of other terms – see below) is the combination of a tractor unit and one or more semi-trailers to carry freight. A semi-trailer attaches to the tractor with a type of hitch called a fifth wheel.

A logging truck or timber lorry is a large truck used to carry logs. Some have integrated flatbeds, some are discrete tractor units, and some are configured to spread a load between the tractor unit and a dollied trailer pulled behind it. More than one trailer is often attached.

The Logistic Vehicle System Replacement (LVSR) is a family of heavy-duty military logistics vehicles of the United States Marine Corps (USMC) based on a common 5-axle ten-wheel drive (10x10) chassis. The vehicles vary in individual configuration by mission requirements, with three variants in service: a cargo, a wrecker and a tractor truck. The LVSR was designed and is manufactured by Oshkosh Defense.

The first LVSRs were ordered in 2006. The LVSR is the USMC's equivalent of the U.S. Army’s Heavy Expanded Mobility Tactical Truck (HEMTT) and Palletized Load System (PLS). The Marines do not use the HEMTT or PLS and the Army does not use the LVSR, but both services use a common trailer (M1076) with all three truck types.

A specialized set of jargon describe the tools, equipment, and employment sectors used in the trucking industry in the United States. Some terms may be used within other English-speaking countries, or within the freight industry in general (air, rail, ship, and manufacturing). For example, shore power is a term borrowed from shipping terminology, in which electrical power is transferred from shore to ship, instead of the ship relying upon idling its engines. Drawing power from land lines is more efficient than engine idling and eliminates localized air pollution. Another borrowed term is "landing gear" (from the aviation industry), which refers to the legs which support the front end of a semi-trailer when it is not connected to a semi-truck. Some nicknames are obvious wordplay, such as "portable parking lot", in reference to a truck that carries automobiles.

An all-wheel drive vehicle (AWD vehicle) is one with a powertrain capable of providing power to all its wheels, whether full-time or on-demand.

Eight-wheel drive, often notated as 8WD or 8×8, is a drivetrain configuration that allows all eight wheels of an eight-wheeled vehicle to be drive wheels simultaneously. Unlike four-wheel drive drivetrains, the configuration is largely confined to heavy-duty off-road and military vehicles, such as armored vehicles, tractor units or all-terrain vehicles such as the Argo Avenger.

A semi-trailer is a trailer without a front axle. The combination of a semi-trailer and a tractor truck is called a semi-trailer truck (also known simply as a "semi-trailer", "tractor trailer", or "semi" in the United States).

A large proportion of a semi-trailer's weight is supported by a tractor unit, or a detachable front-axle assembly known as a dolly, or the tail of another trailer. The semi-trailer's weight is semi-supported (half-supported) by its own wheels, at the rear of the semi-trailer. A semi-trailer is normally equipped with landing gear (legs which can be lowered) to support it when it is uncoupled. Many semi-trailers have wheels that are capable of being totally dismounted and are also relocatable (repositionable) to better distribute load to bearing wheel weight factors. Semi-trailers are more popular for transport than full trailers, which have both front and rear axles. Ease of backing is cited as one of the semi's chief advantages. A road tractor coupled to a semi-trailer is often called a semi-trailer truck or "semi" in North America and Australia, and an articulated lorry or "artic" in the UK.

A 6×4 or six-by-four is a vehicle with three axles, with a drivetrain delivering power to wheels at the ends of two of them. It is a form of four-wheel drive but not one of all-wheel drive.

It is the most common form of drivetrain of semi-tractors and heavy haul fixed-chassis cargo trucks in larger countries such as the United States and Australia; in Europe, 4×2 and 6×2 variants are more commonplace.