Airplane contrails are real, but we know what they are: water vapor  and small amounts of other combustion products created by jet engines.   The other pollutants may be somewhat harmful, but they are basically the  same thing as produced by any other form of combustion.  So what are  chemtrails?  Wikipedia sums it up nicely:

The chemtrail conspiracy theory is the unproven belief that  long-lasting trails, so-called "chemtrails", are left in the sky by  high-flying aircraft and that they consist of chemical or biological  agents deliberately sprayed for sinister purposes undisclosed to the  general public.

When I say chemtrails may be real in the future, I'm really only  making the claim that high-flying aircraft will be deliberately spraying  chemicals that are not simply a normal by-product of jet engine  operation.  However, it won't carried out for "sinister" reasons and it  won't be done in secret.  Instead it will only be done after a great  deal of public discussion and scientific deliberation.

The chemical in question? Sulfur dioxide.

In 1991 Mount Pinatubo in the Philippines produced the second largest  volcanic eruption of the 20th century.  Besides massive amounts of  volcanic ash, Pinatubo injected an estimated 17 million metric tons of  sulfur dioxide into the atmosphere.  This caused measurable global  cooling of about four-tenths of a degree Celsius from 1991 to 1993.  The  effects of the eruption lasted for about three years. [2]

The Mount Pinatubo eruption and its worldwide effect on the climate  has raised the possibility that we could intentionally inject sulfur  dioxide into the atmosphere to counter-act the effects of global  warming.  This kind of large scale intentional intervention is  frequently called "geoengineering" although Wikipedia prefers the term  "climate engineering". [3]
HN discussion: