What could be the problem with your car if it overheats?

What could be the problem with your car if it overheats?
Not knowing much about cars, I know my car used to overheat in the radiator which is usually at the front of the vehicle near the headlights. If there isn't enough coolant in the radiator, or if there is a leak your car will overheat and you will see smoke coming from your hood. A word of caution. Do not open the radiator cap if your car is hot. Wait for it to cool down before opening it because it will explode and you could possibly get burned from the escaping coolant vapors.
The coolant system in most cars is water based (Porsche used to have an air cooling system). So, if you have a car that is overheating it's going to be something to do with the water, or its anciliaries
is there enough water in the system? check for leaks, etc
is the water free to move around the engine (the water absorbs heat from the engine as it passes through the various channels and is then cooled by air hitting the radiator)
so, check that the thermostat is not stuck closed, that there are no blockages in the coolant system and that the water pump is working
Make sure that you don't drive a car that is over heating. Parts of the engine are made from different materials and therefore expand at different rates when the engine heats up. this can lead to head gasket failure and warped cylinder head which is costly to repair.

here are some other reasons from a differant person.

After 5 years of racing i think i can answer this, there is a large variety of simple reasons why a car would overheat, heres the list

blocked radiator- this can be fixed by simply scraping the junk off of your radiator.

low or worn out oil- replace your oil and check for leaks- if there is a leak, replace the leaky part or line.

broken water pump- replace your "water" pump, water or coolant is used to keep most engines cool.

low coolant or water: add more water or coolant to your radiator, once again- check for leaks. (if you choose to use plain water, make sure it is distilled)

worn out engine- your engine may just be wearing out- if this is the case it will need a rebuiled or to be replaced.

faulty sensors- your engine may not be overheating at all, the sensors may just be broken.

broken radiator fan: this is a problem if the car needs artifficial air flow from a fan to cool itself when it is not moving, you can either replace the fan, or get airflow to the radiator some other way- such as driving, or just shutting off your engine before it gets to hot.