What are the white lines left on the sky by jets?

No, these are not typically one man jets. They're commercial aircraft carrying either passengers or air freight, and sometimes they're military aircraft. Any time a hydrocarbon (such as jet fuel) is burned, the waste products are always carbon dioxide and water vapor. The water vapor is quite hot, because it just came out of a jet engine, and the air around it is quite cold due to the high altitude. For this reason, the water vapor condenses and becomes visible, exactly like a cloud. You can actually watch these condensation trails being formed as a plane flies over you, if it's up high enough. Down lower, the air is too warm for the trails to form. I hope this clears up the mystery for you.

Those are contrails or vapor trails caused by airline engine exhaust and wigtip vortices. The principal byproducts of jet engine exhaust are carbon dioxide and water. Because the air temperature at airline cruising altitude is as much as 40 degrees below zero, the water vapor that is expelled from the engines freezes and actually forms little cirrus clouds. If you look carefully at the front of a contrail you can see the airplane causing it.

They are called 'Contrails' or 'Vapour' trails. They're just condensation trails or artificial clouds made by the exhaust of aircraft engines or wingtip vortices which precipitate a stream of tiny ice crystals in moist, frigid upper air. They're behind one of the reasons why we have a lot of pollution.

EDIT: There really is no 'purpose' for the contrails. They are necessary to give off exaust. Like a car. It's the way the jets run and trust me, creating contrails is not just for fun. The last thing we need to make is more pollution.

Vapor Condensation Trails

Contrails.

"The condensation trail left behind jet aircrafts are called contrails. Contrails form when hot humid air from jet exhaust mixes with environmental air of low vapor pressure and low temperature. The mixing is a result of turbulence generated by the engine exhaust."

Artificial clouds, basically. They're vapors left over from the combustion process in the jet engines. They're all the same, but sometimes they hang around longer because of weather conditions.

They are vapour trails, exhaust from jet engines. Like clouds, they gradually dissipate.

contrails that eventually dissipate like clouds.

they're called vapor trails.

vapor form extra fuel they let it off before they land.