The catalytic converter is part of the exhaust system. It's purpose is to "filter out" HC and NO. To keep it simple, its basically a metal housing with metal baffles inside. If these baffles break down, the filtering effect is lessened.

What were your HC and NO levels?

before the new cat, I was getting 180 + on my HC, after it was down to 23...can't remember the NO numbers.

I through on a high flow cat and still passed smog.