A V-twin engine is an
internal combustion engine with two
cylinders arranged in a "V" shape. It's a fairly common arrangement, and nearly every major
street bike manufacturer has a
motorcycle with such an engine.
The torque curve of a v-twin is such that they make better low-end and midrange power than their four-cylinder counterparts, and this is what gives Ducatis their addiction factor. Torque is arguably more addictive than horsepower.
Harley-Davidson motorcycles use V-twins in their bikes, but they're such an archaic design that they're good for nothing but noisemaking.