First, I'll state the obvious, that you should have asked (or googled) before placing your order.
A catback replaces everything *behind* the cats. It'll replace the pipes that go over the axle, mufflers, and tips.
As for loudness, I would put the Pypes MMS between 8 and 9 compared to other catback systems.. They're straight through mufflers, so it doesn't get much louder than that. But with the stock mid-pipe and cats, you won't be waking the dead. The stock mufflers are pretty restrictive, but so are the cats. It'll definitely be a great sound, but don't expect to make people go deaf when you fly past them.