The main difference between cast iron and pig iron is that:

Cast iron is the purest form of iron while pig iron is impure

Cast iron has lower carbon content as compared to pig iron and is extremely hard and brittle.

Pig iron contains many impurities like S, P, Si, and Mn while cast iron does not contain any impurity and can be cast into any shape

Cast iron is soft and malleable while pig iron is extremely hard and brittle