Classical conditioning of the eyeblink response in the rabbit.
A model of a simple predictive system (associative memory).

Rabbits, like other vertebrates, have an inborn eyeblink reflex which can be elicited by a number of stimuli (e.g. tactile stimulation of the surface of cornea and/or peri-ocular region, a bright flash of light).  These stimuli are called unconditioned stimuli (US). 

The problem with the inborn eyeblink reflex is that it is activated by potentially destructive forces only after they could have done damage.  A major evolutionary advancement was made when vertebrates evolved the ability to learn to generate defensive behaviors in anticipation of the destruction.

A simple form of this process is studied in labs using the classical conditioning paradigm. In this task, the subject is presented with a neutral stimulus (or conditioned stimulus, CS, e.g. a tone) which is followed immediately by an unconditioned stimulus.  If this procedure is repeated over and over again, the subject learns that the CS predicts the US, and it develops an anticipatory eyeblink, that is generated just before the US.  This new learned response is called a conditioned response (CR).