Classical conditioning

Classical conditioning (also Pavlovian conditioning) is a type of learning that happens subconsciously. 
Ivan Pavlov (1849-1936) was the first to show the way in which it works. He did this in an experiment using dogs. Pavlov noticed that the dogs naturally salivated when they saw food. This behaviour did not need to be taught. In this example, the food is an "unconditioned" stimulus. He paired this unconditioned stimulus (showing food to the dogs) with another, "neutral" stimulus: the ringing of a bell. Pavlov discovered that, if the two stimuli are presented together again and again, the organism "learns" that they belong together. As a result, it is enough to show the neutral stimulus to get a conditioned response or reflex.
Classical conditioning is an important factor in everyday life. It can be applied in many areas such as behavioural therapies, responses to drugs and in modern day marketing strategies.
Classical conditioning is different from operant conditioning. Operant conditioning is a different kind of learning. It happens when an individual or animal learns a behaviour as a result of a consequence of that behaviour. For example, a child may learn to open a box to get some sweets. So, when a behaviour has a good result, the organism learns to associate the behaviour with this good result. As such, learning the behaviour and repeating it. 
So, operant conditioning explains voluntary changes in behaviour. Classical conditioning explains how reflexes are trained.
Applications.
Behavioural therapies.
The same learning process that develops a conditioned response can be used to teach a new behaviour or change old ones. Some mental health therapies are often linked to classical conditioning. 
Aversion therapy aims to stop a harmful behaviour by replacing a positive response with a negative one. This makes a person associate the harmful behaviour with a bad feeling. For example, a doctor can give a person a medicine that makes them sick (unwell) if they drink alcohol. This person will learn to associate the drinking of alcohol with feeling ill. 
Systematic desensitisation is often used to treat phobias and other anxiety disorders. The person is exposed to what they fear or what causes them to feel anxious. The therapy aims to condition a response of relaxation instead. This is a form of counterconditioning. In time, the feared stimulus produces a conditioned response of relaxation. 
Flooding is similar to desensitisation. This therapy directly exposes the person to their phobias and anxieties. For example, a person who is claustrophobic is locked in a small room for some time. This aims to expose the person to their feared thing in a safe environment and help them overcome their fears.
Conditioned drug response.
There are two kinds of conditioned drug responses. 
First, a drug user may become more tolerant to a drug if it is taken in similar surroundings (a similar setting) every time. This is because the familiar setting will bring about a compensatory response. As a result, taking the drug in similar settings will help your body tolerate the drug. Alternatively, taking the drug outside of the regular setting will make it harder (more difficult) for your body to manage its effects. For this reason, overdose is more common when a user takes a drug in a new setting. 
In other cases, the opposite may happen. If a stimulus is present every time a drug is taken, the stimulus alone may produce a conditioned physiological response the same as the effect of the drug. For example, people who drink lots of coffee may experience the feeling of being alert (wide awake) from the "smell" of coffee. So, without drinking coffee, they experience the same effects from only the smell.
Modern culture.
Classical conditioning theory is used in the modern day to elicit a certain response. A common example is its use in advertising. Advertisers will attempt to make consumers associate their product with a good feeling or response. For example, beauty adverts will use models with clear skin to advertise their products. This will make people associate the product with smooth, healthy skin.
Other examples of strategies used to make a positive advertisement are using happy music, good-looking models and cute animals and babies.