An interactive oracle proof (IOP) is a proof system that combines PCP and IP. An IOP allows both the prover and verifier to send messages like in the interactive proof model, but the verifier is given oracle access to the prover's messages. This is similar to a PCP, but instead of sending one fixed proof, the prover can send multiple proofs during the interaction, with the proofs' contents depending on the verifier messages. The verifier can then randomly query symbols from the received proofs.
The class of problems solved by IOP is NEXP, which is also the class of problems solved by PCP. Therefore IOPs are not more powerful than PCPs, but they can achieve better parameters such as total proof length, alphabet size, and query complexity. The parameters of an IOP are determined by simply summing up the parameters at each round. IOPs can be converted to IPs using Merkle trees via the BCS compiler, the same way PCPs are converted.
No class.