PUL fabric, or Polyurethane laminate, is the most commonly used material for making nappy (diaper) covers. It is very popular largely because it is very water-proof but breathable at the same time. For that reason, PUL is also used in the lining of pocket and all-in-one nappies.
PUL was orginally developed for medical use, where it is still used, as it stands up well to repeated heavy washing and can be effectively sterilised after use.
PUL fabric is non-biodegradable, and many cloth diapering parents prefer to use wool covers, but PUL is still a better choice for the environment than disposable nappies, and generally provides a more reliable moisture barrier.