A nailed plank is an upgraded version of the plank. The nailed plank still takes considerable amount of time to kill a zombie and cannot be used for fortification, so think before you hammer a nail into one of your planks.
A nail plank will not be good for fighting more than 4 zombies. Its long reach is one thing, but its poor damage more than counterbalances its reach. The nailed plank is more useful for crowd control methods, which can be accomplished by the regular plank.
Putting a nail, a plank, and a hammer into the crafting box will give you the nailed plank.