As we know, classical conditioning was discovered by Pavlov. This type of conditioning is also known as respondent conditioning and occurs when two linked stimuli produce a newly learn response. Pavlov discovery was a product of his experiment involving two dogs. He rang a bell immediately before feeding the dogs, and therefore the dogs associate the sound of the bell with the presentation of food. The bell(sound) and the food are the two linked stimuli, and the newly learned (conditioned) response was the dogs salvation after the bell was rung.
There is a perfect example of this in my personal life. My cat loves to loudly meow in my face for attention over night. I began to get out of bed to feed him, as he always quiets down after eating. I noticed after a week or so, my cat know associates loudly meowing in my face with getting fed. I am currently trying to figure out how to uncondition this response, as the quality of my sleep has decreased.
Here is a video with a more information, and description of terms associated with classical conditioning.