Cooling systems are one of the most important systems in an engine. Running an engine too hot can cause fluids and parts to break down an can ultimately lead to a catastrophic failure. At the same time an engine that is running too cold can be inefficient and poor performing.
There are two ways that engines have traditionally been cooled, by air and by water. Many old cars and even some newer motorcycles feature air cooled engines. However, most modern cars require liquid cooled systems.