Its called a H-pipe, because there the pipes from the left header and the right header are connected by a straight balancer pipe just before the mufflers. The resulting shape is a H.
One alternative is an X-pipe. This is similiar to a H-pipe, but instead of using a balancer, the pipes blend together into one large pipe, then spilt apart, forming an X.
Most after-market H and X pipes are offered in off-road versions, and catalitic versions. The off-road version has no cats and are illegal, but you can get away with it if you really want to. The catalytic versions usually have just 2 cats, 1 on each side.
In general, any aftermarket H or X pipe will provide more hp and torque, but H-pipes typically give better low end torque then X-pipes, while X-pipes give better high end hp.
If you bend down and look under the car, you can get a good look at how this is all put together.
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