V-twin engines are two-cylinder engines where the cylinders share a common crankshaft but are not parallel with each other. They are arranged in a V configuration, with separate cylinder heads instead. Most manufacturers elect to mount them transversely — this layout has the crankshaft travel across the motorcycle, allowing the cylinders to be in line with the body, i.e., longitudinally. This allows the motorcycle to be narrower and for the torque generated during acceleration to be distributed along the length of the motorcycle. However, the rear cylinder receives poorer airflow and is thus prone to getting very hot.
On the other hand, longitudinally-mounted V-twins generally have better cooling. This is because the cylinders can protrude outwards on either side of the motorcycle and thus receive better airflow. Another benefit is that it can be easier to implement for shaft-driven motorcycles (where the rear wheel is connected to the transmission via a driveshaft rather than a belt or chain) since the crankshaft travels along the body of the bike instead of across it. One major drawback of this layout is that torque under acceleration can cause the motorcycle to twist in one direction.
The angle between the two cylinders can vary from manufacturer to manufacturer, generally between 40 and 90 degrees. 90-degree V-twins are also known as L-twin engines because the cylinders form an L rather than a V. They are relatively uncommon and generally reserved for racing implementations, like in some of the best Ducati motorcycles.