Reptiles are cold blooded, which means they don't keep their bodies at a constant temperature. If the temperature outside changes, the reptile's body also changes it's temperature. This system doesn't take any energy, so reptiles don't need to eat as much as humans do, and they don't need to breathe as much, either!
Because reptiles don't need as much oxygen as humans do, their lungs are a bit different than ours. Human lungs are very spongy and full of air sacs that can absorb the oxygen out of the air. On the other hand, reptile lungs are less spongy and more hollow, which makes them less efficient at getting oxygen. Another difference is that humans have a large muscle called the disphragm under their lungs, that allows us to breathe in and out. Reptiles don't have this muscle, so they breathe either moving their throat or their ribs.
Since reptiles don't need as much energy as mammals do, they only breathe once in a while. Most of the time, their lungs aren't moving. Then, when their body needs energy, they breathe in and out a few times, and stop breathing again!