Post is the O2 sensor located behind the converter and Pre is the sensor located in front of the converter. Sometimes also referred to as upstream and downstream.
It will create a better flow and your car will run much better without all the back-pressure the cat creates.. But it is illegal in every state and is a huge fine if you get caught. And you WILL get...
Pre is the oxygen sensor that is between the engine and the catalytic converter. Post would be the one after the cat. The pre cat O2 sensor monitors how the engine is managing the air/fuel ratio and...
Direct to fit Catalytic Converters come ready to install with all parts pre-welded. These are better for the DIY professional. Universal converters are cheaper, but require cutting, welding, etc....