RambleOn had a fairly good idea. I would like to add specifics.
Classical conditioning is the pairing of an Unconditioned Stimulus (UCS) with a Conditioned Stimulus (CS) to produce a Conditioned Response (CR).
Pavlov found that that dogs began salivating before the food was actually served. The UCS was the food which produces an Unconditioned Response (UCR) which is salivation. Salivating before the food was served was not as much salivation as if the food was there, which was the CR. He found that the dogs started salivating when they heard the assistants coming. He tested this hypothesis by having a buzzer or light go on just before the food came in, the CS. He found that after a number of trials, the CS alone without serving food produced the CR. If the CS is produced after a number of trials without the UCS, eventually the CR stops, which is extinction.
Classical conditioning differs from instrumental conditioning because in instrumental conditioning a stimulus (S) is said to elicit a response (R) and the likelihood that that R will occur to that S can be strengthened if it is reinforced or weaken through removal of reinforcement or through punishment.