Complex sentence: The popular and significant layout has cylinders arranged in two banks on either side of a single crankshaft and is generally known as the boxer, or horizontally-opposed engine--not be confused with opposed-piston engines, which are mechanically quite different.
Split into simpler sentences: The popular and significant layout has cylinders arranged in two banks on either side of a single crankshaft and is generally known as the boxer. Flat engines are commonly described as horizontally opposed engines but must not be confused with opposed-piston engines, which are mechanically quite different.