In classical conditioning a stimulus that already leads to a response is replaced by a different stimulus. In operant conditioning a behaviour is picked out and either reinforced or punished to make it more or less common. So you could say one acts to modify or replace the stimulus that leads to a given response, and the other to modify or refine a response.
In terms of Pavlov's dog (classical) this meant replacing the smell of food, as the stimulus that leads to salivation, with the sound of a bell instead so that eventually even with the sound of the bell alone the dog has 'learned' to salivate. In terms of Skinner's cat (operant) the stimulus was confinement and the natural behaviour was to explore the environment for an escape. Teaching the cat to press the lever to get out can be seen as refining this behaviour with a reward. Here, of course, the cat was not shown how to press the lever. It discovered it randomly on it's own. But thereafter it did not waste much time before pressing the lever
In classic conditioning (the pavlovian's experiment),
The bell ringing alone can produce the same effect(salivation) after repeated association of bell ringing with the sight of the food because of conditioning.
unconditioned stimulus(sight of food)----results in salivation(unconditioned response)
Conditioned stimulus(bell ringing)---also result in salivation (conditioned response)
The key stimulus is beforethe response. (eg, bell ringing is followed by salivation )
In operant conditioning (the skinner's experiment),
there is learned response (pressing the bars) in order to get food . The behavior of pressing the bar is increased because of the reinforcement (food delivery). The key stimulus reinforcement is after the response.(eg pressing of the bar is followed by getting food )
In classical conditioning- animal learns to associate something which has no meaning to it(bell) to something which is rewarding(food)..
to remember operant conditioning, remember that its also called instrumental conditioning- the animal learns to perform particular behavior(pressing bar) in order to obtain an intrinsically rewarding stimulus(food)...
Just another way of explaining what others have done
Classical conditioning- involuntary response (ringing of bell by human causing an involuntary response of salivation)
Operant conditioning- voluntary response (mice voluntarily pushes the bar to get a voluntary response of food)