Man you really need to learn how to search for answers as your questions have been answered hundreds of times. Our cars have been around for more than 15 years now so you aren't blazing a new trail here. Searching for answers is a forum rule if you didn't know and you can be given infractions if you don't follow them. We don't mind helping out but we also won't spoon feed you if you don't put forth any effort on your part to educate yourself.

However I'll answer your question here. Catalytic converters are part of the emissions system and they help filter contaminants out of the exhaust as it exits the motor. They also muffle the sound coming out of the motor. Test pipes are just straight pipes that do neither. Resonated test pipes do tone down the sound a bit but they are still louder that stock. A lot of times TP's will create rasp and drone and makes your exhaust sound like crap. You can get HFC's (look that up) as well instead.

 

lmao