A heater core is a device which is used to generate warm air to heat the inside of a car, truck, or boat. Heater cores can be used with any vehicle which has a liquid-cooled engine, with air-cooled engines requiring a different method of heat generation. People who have noticed that their car heaters don't seem to be putting out as much heat as they used to may want to ask a mechanic to take a look at the heater core; it may need to be repaired or replaced.
The design of the heater core is similar to a radiator. Liquid from the engine passes through a series of tubes and fins in the heater core which are designed to dissipate heat rapidly so that the fluid, which is extremely hot from passing through the engine, will cool down. As the liquid moves through the device, air passing over or around the device picks up the heat, and this air can be directed into the cabin.
While a heater core alone cannot sufficiently cool engine coolant, it is part of the system which keeps engine coolant recirculating through the engine to prevent the engine from overheating. It also creates a way to use engine heat efficiently; rather than just venting the heated air, the heater core directs the air into the cabin so that people can be warm.
Obviously, people don't necessarily always want hot air being blown into the passenger compartment of a vehicle. The amount of air from the heater core which is allowed to enter the vehicle can be adjusted by turning knobs which open flaps to allow varying levels of air into the cabin. Passengers and drivers typically have a temperature dial which allows them to select a comfortable setting, along with fans which can be used to blow air into the cabin, and they may also be able to select the locations where air enters the cabins, opting to only have air blowing from the dash, for example, or to turn foot heaters on as well.
The tubes in a heater core can become clogged or bent, impeding the flow of fluid and reducing the amount of heat which can be transferred to the air which flows around the heater core. Heater cores can also develop leaks which create a problem by allowing the car to lose coolant and cause a similar lack of heat transfer. People who are experienced with cars can often replace their own heater cores, while people who are not comfortable with dismantling their dashboards may need to see a mechanic.