Dogs and Cats (like humans) have bacteria in the gut. The diet you feed them can help manage the types of bacteria to produce a more favorable environment for overall health. Prebiotics can play an important role because they help nourish the naturally occurring probiotic bacteria that is in your pet’s gut. Here is the difference between prebiotics and probiotics:

Prebiotics: A non-digestible food ingredient, (such as fiber) that supports a favorable environment for bacteria and cells in the gut wall of the digestive tract. Prebiotics enable the growth & activity of bacteria that populate the gastrointestinal tract. Prebiotic fibers ferment during digestion and grow bacteria in the gut (digestive tract). These bacteria add to any probiotics that naturally occur. 

Probiotics: Probiotics are a dietary supplement containing live bacteria that adds to the bacteria normally present in the gastrointestinal tract. They are living organisms that must remain alive in order to work. Probiotics can be diminished or even die completely when exposed to heat, long periods of time, and acidity. If you see products with probiotics added, there is a possibility that by the time you use it, the probiotics may be dead and offer no digestive benefit to your pet.