Usually consists of a "Y" pipe (or pipe directly at outlet of catalytic converter(s)formed to reduce turbulence and thus maximize flow and maybe with increased inside diameter such as 2 1/2" instead of 2 1/4".
Includes low restriction mufflers which would probably produce louder exhaust noise than stock.
There are many claims to horsepower gains which state "up to xx horsepower increase". While increased horsepower is the goal and is likely, the increase might also be exaggerated by drivers due to the louder sound associated with more power.
Too much exhaust volume flow in some vehicles can actually reduce low speed-torque. ref. Chevy High Performance magazine, Aug 2004, Q&A pg. 136.