I'll touch on that topic with a rather basic explanation.

Imagine a champagne bottle, if you pop the top off rather quickly it'll send the cork flying and create that "pop" noise. Now if you slowly inch the cork up allowing the gas to slowly relase you won't get that "pop" noise.

Basically a supressor is designed with baffles (or vortex chambers) in which the bullet travels thru. Each of these chambers capture the explosive gas throughout it's flight through the can.

A really good article with pictures, some with a actual supressor broken down exposing the baffles can be seen here...
http://www.mark23.com/NGNBTD/silencers.html

Remember to use subsonic ammunition, to increase the effectiveness.