In classical conditioning, there has to be a natural (unconditional) response to a natural (unconditional) stimulus, For instance, when a hungry dog is presented with food (unconditional stimulus), it will salivate ((unconditional response). The idea is to make it salivate to a new stimulus - say, ringing of a bell- by association of the bell -ringing with food.
In operant conditioning there is no unconditional stimulus and no unconditional response. Instead of these a response that the experimenter desires is made to occur by reinforcement (reward) or by punishment. For instance a rat can be made to choose a particular path in a maze by giving it an electric shock every time it chooses the wrong path.