In this paper we investigate some properties of zero-knowledge proofs, a notion introduced by Goldwasser, Micali and Rackoff. We introduce and classify various definitions of zero-knowledge. Two definitions which are of special interest are auxiliary-input zero-knowledge and blackbox-simulation zero-knowledge. We explain why auxiliary-input zero-knowledge is a definition more suitable for cryptographic applications than the original [GMR1] definition. In particular, we show that any protocol composed of subprotocols which are auxiliary-input zero-knowledge is itself auxiliary-input zero-knowledge. We show that blackbox simulation zero-knowledge implies auxiliary-input zeroknowledge (which in turn implies the [GMR1] definition). We argue that all known zero-knowledge proofs are in fact blackbox-simulation zero-knowledge (i.e. were proved zero-knowledge using blackbox-simulation of the verifier). As a result, all known zero-knowledge proof systems are shown to be auxiliary-input zero-knowledge and can be used for cryptographic applications such as those in [GMW2]. We demonstrate the triviality of certain classes of zero-knowledge proof systems, in the sense that only languages in BPP have zero-knowledge proofs of these classes. In particular, we show that any language having a Las vegas zeroknowledge proof system necessarily belongs to R. We show that randomness of both the verifier and the prover, and nontriviality of the interaction are essential properties of non-trivial auxiliary-input zero-knowledge proofs. In order to derive most of the results in the paper we make use of the full power of the definition of zero-knowledge: specifically, the requirement that there exist a simulator for any verifier, including "cheating verifiers".