The cooling system is designed to remove heat from the engine to keep the engine operating
in it's optimal temperature range. The cooling system is comprised
of the following parts...

Water Pump: The water pump is either driven by the fan belt or the timing belt on some newer vehicles.
The pump moves coolant from the radiator, through the engine, and
back into the radiator. The pump has a shaft with a pulley on one
end and a pump rotor on the other end. When the pulley is spun by
a belt, the rotor moves the coolant.

Radiator: The radiator is a
series of thin channels where the coolant flows through and it cooled
by air flowing through it. It is important to keep your coolant
in good condition to keep the channels open in the radiator. Following
your manufacturers recommendation for changing your coolant, to keep your
radiator in good shape.

Thermostat: The thermostat controls the flow of coolant through the engine. When your engine
is cold, it actually operates with less efficiency. So until the
engine warms up to it's ideal temperature, the thermostat keeps coolant
from flowing. Your thermostat can fail in either an open or closed
condition. When it fails in the open position, your engine may actually
run too cold and you will have bad gas mileage. If the thermostat
fails in the closed position, your engine will overheat since no coolant
will be flowing through the engine.

Hoses: The hoses carry the coolant
from the radiator to the waterpump and from the engine to the radiator.
Hoses are generally made of rubber and can deteriorate with time.
Hose should be flexible and not dry rotted (check for tiny cracks in the
hose)

Coolant:
Coolant is generally a mix of water and ethylene glycol. Many times
called Anti-Freeze, coolant serves many purposes. As it's common name
implies, coolant prevents freezing, but it also provides lubrication
for the waterpump, increases boiling point of water, and keeps rust and
scale from forming in your cooling system. Coolant must be changed on
a regular basis, check your owners manual for the recommended
schedule. PH level is critical to keeping your coolant from becoming
a metal eating liquid. When you check your coolant for freeze point,
also check out the PH level and make sure your coolant has not turned
acidic.

Common Problems:

Let's look at the common
problems cars have with the cooling system.

• Broken hose. Hoses wear out and can leak.
  Once the coolant has left the system it can no longer cool the engine
  and it overheats.
• Broken fan belt. The water pump is driven
  by the engine through a belt. If the belt breaks the water pump can
  not turn and coolant will not be circulated through the engine. This
  will also lead to engine overheating.
• Faulty radiator cap. The radiator cap is
  designed to hold a certain pressure in the coolant system. Most caps
  hold 8 - 12 PSI. This pressure raises the point in which the coolant
  will boil and maintains a stable system. If your cap does not hold pressure,
  then the car could overheat on hot days since the system never becomes
  pressurized.
• Water pump failure. Most commonly you will
  hear a screeching noise and will be able to see coolant leaking from
  the front of the pump or under the car. Early signs are small spots
  of coolant under the car after being parked overnight and a strong coolant
  odor while driving.
• Head gasket... have large amounts of white
  smoke flowing out of your exhaust? Could be a head gasket. The head
  gasket seals the cylinder head to the engine block and also seals the
  coolant passages. When this gasket fails coolant can enter the cylinder
  and it will be turned to vapor as the engine fires. Head gaskets most
  often fail after the engine has experienced an overheating situation.
  When super hot, the cylinder head can warp and allow the gasket to fail.

Preventive Maintenance:

• Check all belts and hoses regularly. (at oil change
  is a good time)
• Look out for coolant leaks underneath the car,
  they could be signs of trouble to come.
• Change your coolant every 2 - 3 years depending
  on the manufacturers recommendations. You can check the freeze point of your coolant with an Anti Freeze Tester available from www.toolsurge.com.
• Inspect your radiator cap for deterioration of
  the rubber seal. Replace if you think it is worn. $5 - $10 is cheap
  insurance.
• Have your coolant system flushed every 5 years.
  It gets all the corrosion which has built up out of the system.

What to discuss with your mechanic:

• Let your mechanic know when your overheating problems
  occur. Overheating when idling points to a different problem than
  overheating at highway speeds.
• Ask your mechanic if it's worth changing the timing
  belt or chain while he is replacing your water pump. Many times
  the timing belt turns the water pump so it has to be removed anyway
  to access the water pump.

WARNING: Never open your radiator when the engine
is hot. The pressure in the system can cause hot coolant to splash out
and burn you.

source:www.AutoEducation.com