I'm not too good at explaining these type of things (i would make a horrible teacher)..so if you would like to learn about the exhaust systems, get a firm grasp on how they work, check out this section of How Stuff Works, and read the catagories on "How a Catalytic Converter works." and "How Mufflers Work". They should help out
http://www.howstuffworks.com/category.htm?cat=EngSys
Hope it helps!
errr.. then again, looking over it, doesnt really help you out on what you need to buy.. oops