Chemtrails is the name given to the unnatural contrails from planes. Contrails are formed by condensation of water vapor in the aircrafts' exhausts. Chemtrails are modified contrails that allegedly have chemicals in them. These chemicals would be used for a wide range of purposes, like weather modification, mind control, population control (!), biological warfare, and even occult purposes. Some other claims are that they would be part of a system to counter the effects of global warming, to create a cheap wireless communications network for the military, or to create a more sophisticated radar system (for both defensive and scientific application). (Source: Wikipedia)
Most of the information on chemtrails on the Internet is from dubious sources.