Contrail !!!!?

I dint come across this word before … untill i got amazed and then pulled into my childhood …. and then into a question to myself. Why don’t I know yet, what causes some Airplanes to leave a cloudy trail?
During my childhood, I got amazed on how some of the airplanes do that and some do not. Even thought that the one with the trails are actually "spacecrafts", just launched. Later I realized that India is still not that forward in space research that we can launch a spacecraft or “Rocket” every other day :P
So, I was roaming in the society, where I stay and I got this …


really liked that … and clicked that from my mobile, thats the reason of the poor quality of the image, but still it luks gr888 ..
So, that day i thought …. why i am still unaware of this, why these airplanes are trailing some …. clouds etc? … Assuming, Google and Wikipedia were not so popular things in my childhood, I actaully forgave my childhood me. But, now this is bad … real BAD …
So, i got into wikipedia and got this - Contrail (Wikipedia) and some interesting answers from yahoo -Myserious trail ( Chemical Spray ??)
So here it is … i got it now .. :) such a relief !!!