The classical conditioning theory was proposed by Ivan Pavlov in the early part of the 20th century.
The gist of the theory is that behavior can be learned by instinct with the application of a positive stimulus. This stimulus is only provided when the behavior is desired. Therefore, the behavior will be produced each time the stimulus is present.
However, there are some variables that must be taken into consideration when applying a stimulus to entice behavior. First, is the duration of the exposure to the stimulus. It has to be done at the appropriate times to ensure that the proper combination of factors.
Once the connection between the stimulus is made, the behavior is more likely to occur. Behavior will likely persist if a conditioned behavior continues to be stimulated. However, if it is not, it may become extinct. Similarly, the theory states that there are rare times when conditioned responses that went into extinction suddenly relapse. This is known as "spontaneous recovery".
Finally, another manifestation of classical conditioning can be that the individual performing the behavior may connect all things related to the stimulus. This is called "generalization": When one reacts the same way to things connected to a specific stimulus. An example of this would be getting either attached to, or scare of, something that reminds us of a stimulus that caused a behavior.
The main difference between classical and operant conditioning is that the latter involves the compensation or punishment of a behavior for it to persist or desist. Contrastingly, classical conditioning is a connection between stimuli and behavior that would ensure that it occurs each time the stimulus is present.