Chemtrails - what are they?

Chemtrails have been appearing all over the place! It appears the skies are being deliberately polluted with chemicals which are harmful to us! Why? By whom? As citizens we should be concerned and we should be asking questions but it appears those who do get no answers. It would be interesting to see if anyone can shed any light on this matter.

Here's what a google search brings up:

http://video.google.com/videosearch?q=chemtrails&sitesearch=#

I've watched a few things on this and I would recommend others to start spreading the word. Broadening our knowledge base may create the necessary pressure to get some answers. People's health is being affected, apparently.