npp is nandrolone propionate with a half-life of 3 days while deca nandrolone undecoanate with a half-life of 15 days.
In the bloodstream, esterase enzyme strip away the ester (propionate or decoanate) and leave the nandrolone free to go about it's anabolic/androgenic business. The main difference is in how fast it will "kick-in", how often you need to inject, and how long it will stay in your system post-cycle.