Probabilistic checking of proofs; a new characterization of NP

The authors give a new characterization of NP: the class NP contains exactly those languages L for which membership proofs (a proof that an input x is in L) can be verified probabilistically in polynomial time using logarithmic number of random bits and sub-logarithmic number of queries to the proof. This is a non-relativizing characterization of NP. They discuss implications of this characterization; specifically, they show that approximating clique (or independent set) is NP-hard