The difference between a hog and a pig is poundage.
In the United States, any swine below 180 pounds is called a pig; a pig that grows larger than that is called a hog.
In England, all swine are considered pigs, whatever their weight.
There are other specialized names for pigs in their various stages of life as well.
At birth they are piglets, weaning after age two to five weeks.
A young pig is called a shoat or weaver.
A half-grown pig is a barrow (castrated male) or gilt (female).
Adults pigs are boars (uncastrated males), stags (castrated males), or sows (females).