Why do those long, white clouds form behind jets?

These long white clouds you are referring to are called condensation trails or contrails. Jet fuel is made of carbon and hydrogen which burns in the presence of oxygen. In this reaction some of the carbon joins with the oxygen to make CO2 (carbon dioxide) and some of the hydrogen joins with the oxygen to make H2O (water). The water which comes out of the engine is invisible water vapour but as soon as it is exposed to the colder temperatures of the upper atmosphere, it condenses into little drops of liquid water which become suspended in the air which is what we see as contrails.
This is a similar effect which you notice on a cold day when you breathe out; the water vapour in your breath condenses as it meets the cold air and you can see the breath.