Towbars are mechanical linkages between two vehicles; through these devices one vehicle provides locomotion and direction to the other vehicle. These are most commonly used in conjunction with roadway vehicles, but railroad versions exist as well. The most notable type of towbar would be used to transfer kinetic energy between an aircraft and an aircraft tug. The towbar attaches to the aircraft's forward-most landing gear for steering. Automotive towbars connect two autos and are less robust that aerospace models. Railyards sparingly use towbars to arrange rolling stock.
Aerospace towbars are almost exclusively aircraft-grade aluminum or steel in construction to meet or exceed the working load. Not only must the towbar be able to safely pull/push the vehicle, it must also be able to withstand the towed vehicle's inertia. Aerospace towbars often have integral wheels; a jack to accommodate various linkage heights; and a shear pin to prevent damage to the vehicles in the event of mechanical failure or overload. Railroad towbars resemble aerospace versions but are meant to interconnect with railcar couplings. Automotive towbars are used to pull other automotives; are rarely used to 'push' other autos; and forgo shear pins due to the recurring nature of auto traffic on roadways.
Video credit: Aero Specialties via YouTube