The standard pigman (not to be confused with zombie pigmen) is the Bipedual relative of the pig obtained by using a nature scroll on a pig. The pigman poses no harm to people but is impervious to your attacks, no matter how vicious; however, the amount of damage the attack would have dealt is displayed when they are attacked. They also have a nasty habit of despawning VERY quickly.
Pigmen and pigs
The Pigman utilises an unused texture within the minecraft.jar files that was origionally going to be for the villagers.