Let's look at the two theories first.
1. Classical or "Pavlovian" Conditioning
Classical Conditioning is the type of learning made famous by Pavlov's experiments with dogs. The gist of the experiment is this: Pavlov presented dogs with food, and measured their salivary response (how much they drooled). Then he began ringing a bell just before presenting the food. At first, the dogs did not begin salivating until the food was presented. After a while, however, the dogs began to salivate when the sound of the bell was presented. They learned to associate the sound of the bell with the presentation of the food. As far as their immediate physiological responses were concerned, the sound of the bell became equivalent to the presentation of the food.
* Stimuli that animals react to without training are called primary or unconditioned stimuli (US). They include food, pain, and other "hardwired" or "instinctive" stimuli. Animals do not have to learn to react to an electric shock, for example. Pavlov's dogs did not need to learn about food.
*Stimuli that animals react to only after learning about them are called secondary or conditioned stimuli (CS). These are stimuli that have been associated with a primary stimulus. In Pavlov's experiment, the sound of the bell meant nothing to the dogs at first. After its sound was associated with the presentation of food, it became a conditioned stimulus. If a warning buzzer is associated with the shock, the animals will learn to fear it. (source = http://www.wagntrain.com/OC/#Operant)
Classical conditioning is very important to animal trainers, because it is difficult to supply an animal with one of the things it naturally likes (or dislikes) in time for it to be an important consequence of the behavior. In other words, it's hard to toss a fish to a dolphin while it's in the middle of a jump or finding a piece of equipment on the ocean floor a hundred meters below. So trainers will associate something that's easier to "deliver" with something the animal wants through classical conditioning. Some trainers call this a bridge (because it bridges the time between when the animal performs a desired behavior and when it gets its reward). Marine mammal trainers use a whistle. Many other trainers use a clicker, a cricket-like box with a metal tongue that makes a click-click sound when you press it.
For example, you can classically condition with a clicker by clicking it and delivering some desirable treat, many times in a row. Simply click the clicker, pause a moment, ...
This solution compares classical conditioning and operant conditioning procedures, and then discusses which learning process is more effective and why in words. Supplemented with one diagram of conditioning procedures and external sources.