difference between soundness in ZKPoK and special soundness for sigma proofs What is the difference between 
soundness in ZKPoK and 
special soundness for sigma proofs?
Soundness usually means "you can't prove a false statement". There are different ways to formalise this but usually the probability of an efficient algorithm coming up with a false statement and a proof that verifies is negligible in some parameter (such as the length of the statement). Soundness can be defined for any proof scheme, including ones that are not sigma protocols.

Special soundness is a particular property of sigma protocols. It implies soundness (the chance of making a false proof is at most the inverse of the size of the challenge space) but it implies a lot more, as it can be used to show that sigma protocols are proofs of knowledge.