Learning

Learning is essential in our everyday life. Everything that we encounter in our daily life is learning as long as it cause you to change. To put in useful definition, learning is any relatively permanent change in behavior brought about by experience or practice.
Dr. Ivan Pavlov discovered Classical Conditioning which is a kind of learning that involve reflex (involuntary response) and stimulus (anything that trigger the response). He first discovered it by experimenting it with the salivating dogs.

Based on the diagram above, there are several key elements that must be present and experienced in a particular way for conditioning to take place.
Unconditioned Stimulus (UCS): the naturally occuring stimulus that leads to involuntary response. In the case of Pavlov's dogs, the food is the UCS.
Unconditioned Response (UCR): involuntary response to the UCS. In Pavlov's experiment, the salivation to the food is the UCR.
Conditioned Stimulus (CS): If neutral stimuli (NS) is paired with the UCS often enough, it will turn into CS. After being paired with the food so many time, the bell came to produce a salivation response,
Conditioned Response (CR): The response that is given to the CS is not usually quite as strong as the original UCR, but it is essentially the same response. However, because it comes as a learned response to the CS, it is called the CR.

Extinction happen when the absent of UCS after such a lost time, and the CS start to be dysfunction and CR will not happen.