what causes smoke at the exhaust of a car,is it just due service or what could be the cause in a petrol engine

You most likely have;
1.Worn piston rings which are allowing some engine oil into combustion chambers thus "polluting' your fuel and hence the black smoke.
2.Poor fuel-air ratio and thus your car fuel system requires adjustment or calibration.

The first case can be determined by a simple test.Start your car and go to the exhaust.Try to choke your engine by holding your palm firmly against the exhaust exit.If the car shudders and shows signs of stalling,your rings are Kaput.If on releasing the palm,the car comes alive and sighs with a burst of "water" and white smoke,your problem lies in the second point....good luck.

@Njung'e sorry to contradict you man - #1 - if rings are worn, smoke would be white (hence the name "crop duster" for cars that are on their last legs). If its dark smoke - then it is burning rich which is your #2 point - that is correct.

Another cause for whitish smoke is a blown head gasket as Jasonhill states - the head gasket keeps the oil and water channels channels separate - if it burns (owing to overheating) then you also get white smoke and you will notice idling is erratic. This kind of over heating can be caused by no coolant, air lock in the cooling system, poor ignition timing, brake jam, driving with the handbrake on (stop blushing ladies

If the smoke is whitish only after starting the car...for a minute or so...then its the valve seals that are worn. When the car sits idle, a little oil at the tappets/rockers area slowly seeps through the worn seals into the cylinder so when you fire up, it burns and then burns out.

Aside from poor fuel/air mixture, I can only think of two other causes of black smoke: dirty carburettor (if its not EFI) and contaminated fuel.

And just one more thing - the exhaust "compression test" Njung'e has described....should only be for a second or two...and not repeatedly for a prolonged period - otherwise you will compromise the rocker cover rubber gasket since pressure buildup there will force it from its seating and the next thing you will have an oily engine and bad smoke when it spills onto the exhaust manifold.

@The Merchant - no I am not a mechanic but take an interest in such things and have learnt from experience...

In addition to very professional advise from Njung'e Jason & Djinn, consider the following:

1. How is the air cleaner/filter? This is a basic cause of smoking?

2. Has the fuel consumption gone up? If yes, refer to number 1 above

3. Is the vehicle lacking power? Points to overheating and blown gasket.

4. Is the vehicle supercharged but still smokes and lacks power? Supercharger is the equivalent of Turbo charger, ama Kompressor ile ya Mercedes. If its supercharged and its smoking, and lacks power, the supercharger or kompressor could be faulty.

@ Djinn, point of correction. Driving with hand brake on can never cause overheating of the engine, only overheating of the wheels/tyres/brakes. Same to brake jam.

@ Simonkabz,
The engine will labour n if the cooling system is faulty, the gauge will "climb" (geji itapanda!). If the cooling system is working fine, there is no way the engine will overheat as you have rightly said it here.

...You mean people drive with their handbrake on?
weird!!!
I think sometimes we are to blame for this overheating stuff . Cars don't just overheat...its necessary to inspect your thermostat, engine coolant sensors,coolant levels, oil levels,whatever the cost.
Some of us go for funny shortcuts doing away with the thermostat then after a couple of years you complaining about worn out engine parts!!!