http://en.wikipedia.org/wiki/Chemtrail_conspiracy_theory
"The chemtrail conspiracy theory holds that some contrails are actually chemicals or biological agents deliberately sprayed at high altitudes for a purpose undisclosed to the general public. Versions of the chemtrail conspiracy theory circulating on the internet and radio talk shows theorize that the activity is directed by government officials.[1] As a result, federal agencies have received thousands of complaints from people who have demanded an explanation.[2] The existence of chemtrails has been repeatedly refuted by government agencies and scientists around the world.[3]"
I don't know anything about this stuff, but one thing I'm sure about is that I saw extremely unusual contrails several years ago. I was visiting Yosemite National Park in October 2001. I was there for three days, and every day parallel contrails would gradually appear overhead, and then eventually the same thing would happen, 90 degrees removed, forming a grid pattern. Whatever was forming these contrails was so high up that we could neither see nor hear it (them?). I was with three other people, and we all saw them on all three days. None of us had ever heard of this "chemtrail conspiracy" at the time, and none of us had ever seen anything like a grid of contrails before. And I, for one, have never seen anything like it since. This was just a few weeks after 9/11, and it was kind of eerie to see something bizarre in the sky like that, which must have been done by planes.