essentially, waht sound deadening does is change the frequency that your cars sheet metals and what not resonate at...
by doing this, they don't buzz and rattle as easily...
also, they provide a physical barrier for the sound waves at teh sheet metal... by providing this barrier, the sounds are less audible outside the car.

*gasp*...
you put it on any piece of your car. the trunk, the roof, the doors, the floor, you name it, you put it there... (in a perfect world)

Edmonton eh? which part, how old are you, etc.