There are two types of fans, mechanical and electric.
Mechanical Fans
Mechanical fans draw more air than electric fans. When a mechanical fan is removed from an engine, it frees up several horsepower. You will not find an electric fan which a motor that produces that much power.
Mechanical fans are driven by the engine. They attach to the engines water pump with either a spacer or a fan clutch. Fan clutches work by a viscous fluid inside them that increases resistance on the clutches as temperature increases. Thus, a cold engine's fan does not spin as fast as when it's hot. Also, the clutch allows the engine to turn a much higher rpms than the fan preventing some parasitic power loss. On deep water crossings, a fan clutch will allow the fan to slow or stop spinning, limiting the amount of water splashed around the engine bay.
Fans that attach to the water pump with only a spacer use flexible blades to compensate for changes in rpm. The blades of a flex fan are made out of a spring material and flatten out at higher rpm so that they maintain the same air flow out all rpm. They are always spinning the same speed, so a cold engine will take longer to warm up. Flex fans are often used when there isn't space for a fan clutch.
