A cat-back just refers to how much of the exhaust is replaced. In this case from the ca[alytic converter]-back is replaced. Along the bottom of the graphic above you can see the axle- cat- and turbo- back exhausts.
It helps by giving you less back pressure, which can give you more power. However, the cat-back doesnt give you that much gains when you're mostly stock. At that point, it's usually for looks/sound.