This is truly the question I am asked most frequently and it is confusing. Probiotics are what most of us are familiar with. Probiotics also are often referred to as Direct Fed Microbials. These are live microorganisms (typically lactobacillus strains) that are included in supplements and yogurts. Products such as Prostora (by Iams) contains live microorganisms so they are considered Probiotics.
In contrast, Prebiotics are food ingredients that selectively stimulate the growth of healthy bacteria such as bifidobacteria and lactobacillus strains. The major groups of prebiotics we refer to as MOS (mannan-oligosaccharides), FOS (fructo-oligosaccharides), COS (chito-oligosaccharides), and GOS (galacto-oligosaccharides). These are all forms of carbohydrates that are not digested by mammalian enzymes in the digestive tract; therefore, they reach the large intestine intact and healty microbes really like them. I tend to like feeding prebiotics more than probiotics because we know these ingredients make it to the large intestine. In my previous post I relayed a few of my concerns (particularly antibiotic resistance) in relation to probiotics. The use of prebiotics does not pose those same risks.
One of the most common prebiotic ingredients I see in petfoods is Chicory Root. Chicory Root contains a high concentration of the prebiotic called Inulin. This compound aids the growth of bifidobacteria in the large intestine. Most people do not need to supplement their pet's diets with prebiotics as they are already included in many of the formulations of high quality petfoods.