Start with the layer of shock-absorbing silicone — rubber, really — that fits around your phone. Presumably it'll help against dust and moisture, too, though the Galaxy S5 is already pretty good at that on its own thanks to its IP68 rating. On top of that rubber skin goes a hard plastic shell — let's just call it an exoskeleton, 'cause it sounds cooler — that not provides even more protection. Some of the rubber skin is exposed, giving you some nice grip points.
There are holes for the microphones and IR port, and a large cutout for the camera and rear speaker. There are rubber flaps that go over the 3.5mm headphone jack and the (already sealed) USB ports, but they're a wee bit flimsy, and we worry a little bit about their longevity. (The headphone jack flap in particular has a habit of not always seating itself correctly.)
Otherwise, no real complaints here. You get a some extra bulk, of course — this is a bit of a tank-like case — but it's not all that bad. Just take your time when seating the rubber around the phone, and again into the plastic shell, and you should be good to go. And if you want, there's also a screen protector (and cleaning cloth) included with the case, so you'll be protected from all sides.