A contrail is a series of clues that lead the police to catch a criminal.

Actually, that’s not the only answer.  Contrails are the trails behind aircraft that look like smoke but are really just water vapour.  Here’s a post I made on the topic on the excellent Taking Flight  fear of flying support forum…

At take-off and landing the wingtips, flaps and any other part of the plane sticking out into the airflow can cause vortices (a spinning spiral of air, causes when air flows from a high pressure region to a low pressure region). The air pressure in the middle of a vortex is reduced, which also lowers it’s temperature. If the humidity is just right, water droplets will form as the water vapour in the air condenses, and you will see a “contrail” (condensation trail).It needs to be fairly humid for this to occur… meaning that there is already a lot of water vapour in the air, and you are not far away from getting rain, fog, clouds forming in the atmosphere. This is because although the air pressure and temperature inside the vortices is reduced, it doesn’t reduce a lot. The water vapour in the air has to almost be ready to condense of it’s own accord.

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At high altitude the contrail is due to the fact that hydrocarbon combustion (the burning of jet fuel, for example) produces water (H2O) as a product. That water is being introduced into the very cold air around it (as vapour or gas) and, if the temperature and humidity of the air are just right, that vapour can condense and freeze into tiny ice particles. Again, a “contrail”.

In the old days jet engines DID produce a lot of smoke, and they were fairly inefficient. These days the quest for fuel efficiency and environmental awareness has produced engines that generally don’t produce much in the way of visible exhaust (other than the contrails described above).

For anyone interested in a little more detail… 
“Contrails” on Wikipedia
“Combustion” on Wikipedia