Describe Pavlov's classical conditioning experiment. (e.g., "Pavlov's dogs"). Part 2: Briefly describe the difference between positive and negative reinforcement

Classical conditioning was conceptualized by Ivan P. Pavlov. Classical conditioning is also known as respondent conditioning. 

Classical conditioning is a type of learning that includes alliance of 2 stimuli. One stimulus is called the unconditioned stimulus (UCS) and the other is called the conditioned stimulus (CS). The UCS and CS are paired together and presented in a number of trials so that conditioning or learning takes place. When a neutral stimulus (NS) is presented alone in the beginning, it evokes no response. The unconditioned stimulus (UCS) is a stimulus that generates response organically or naturally and is not based on any previous conditioning or learning whenever it is presented before the organism. The unconditioned response (UCR), is the response that occurs spontaneously or automatically without any kind of previous learning. 
When the neutral stimulus (that evoked no response when it was presented for the first time) is continuously paired with the UCS, it procure the features of UCS and generates an unconditioned response. So, later when the previously neutral stimulus is presented alone without the UCS, it produces a response similar to the response that  is evoked when the unconditioned stimulus is presented. Thus, conditioned stimulus (CS), is the stimulus, that was initially not capable of generating any response, but later after being paired with the UCS, this generates a response similar to the UCR, when presented alone. This response which is generated when the conditioned stimulus is presented is called the conditioned response (CR). Thus, CR is the response evoked when the CS is presented alone and this response is not natural or spontaneous, but is learnt and is based on experience.

Pavlov's experiment on dog laid the basis of classical conditioning. He was initially studying the process of digestion on dogs. While measuring the amount of salivation of dogs, he noticed, that the dog often salivated when it smelled food or even when it viewed the empty food pan. This observation, that the dog started salivating even before it actually tasted the food instigated Pavlov to conduct a study.

Pavlov began the experiment, by presenting a neutral stimulus, which was a bell that caused no salivation in the dog. But when food was put in front of the dog, the dog salivated. This response evoked by the dog is spontaneous or natural. The food is known as the unconditioned stimulus (UCS), and the response evoked at the sight of the food, which is salivation is known as unconditioned response (UCR). The stimulus food is known as UCS as the response given while presenting the UCS before the organism is not contingent upon any kind of learning, the UCR is unlearned.

During the conditioning trials, Pavlov, paired the sound of the bell along with the food. This  pairing was done for a number of times and presented before the dog. After this continuous pairing, the previously neutral stimulus (bell), acquired the traits of UCS (food). Now, the dog started salivating to the sound of the bell only. The neutral stimulus, the  sound of the bell at the beginning of the experiment, failed to elicit responses in the form of salivation in the dog but now it succeeded in eliciting the response (salivation) in dogs alone after being repeatedly paired with the UCS  (food).  So, now the dog salivates as soon as it hears the sound of the bell even without food. Hence, the previously neutral stimulus, bell is now known as conditioned stimulus (CS), and the response evoked only at the presentation of CS is known as conditioned response (CR).

Neutral stimulus (sound of the bell).......... No response 

UCS (food)........ Salivation (UCR)

CS (sound of the bell) + UCS (food)........ Salivation (UCR)

CS (sound of the bell)...........Salivation (CR).

This explains Pavlov's experiment on classical conditioning.