"What's the difference between a probiotic and a prebiotic?"
This is a GREAT question. Terms sound very much alike, yet mean completely different things. In as short of an answer as possible, a probiotic provides the gut with bacteria, in contrast to a preobiotic, which provides food for already existing bacteria in the gut.
A longer, more elaborate, and substantiated answer follows:
According to the WHO, a probiotic is a "Live microorganisms which when administered in adequate amounts confer a health benefit on the host."
Recent studies with probiotics have indicated tremendous health benefits, far beyond merely reestablishing bacterial balance in the intestine. In fact, this Mayo Clinic page says that "probiotics are ... are sometimes used for gastrointestinal conditions such as diarrhea and irritable bowel syndrome. They may also help your body digest lactose."
"I'm a great believer in probiotics," says Walter J. Coyle, MD, director of the Gastrointestinal Program at Scripps Clinic Medical Center in La Jolla, California "I definitely recommend them for irritable bowel syndrome, regulation, and bloating."
I'll leave it up to you to do a search for probiotic information as we are legally obligated to not mention specific diseases. So go to your favorite search engine and look for something interesting.
A prebiotic is a food ingredient that the body cannot digest and somehow stimulates the growth or activity of bacteria. Essentially, it's in your food but it doesn't feed you, it feeds the your intestinal flora. These are normally medium-sized chains of carbohydrates (scientifically known as oligosaccharides). Hmm.. what do I know of that contains oligosaccharides? Oh yeah! Our glyconutrients supplement! Right.
To summarize, a probiotic adds organisms to your gut; a prebiotic add food for organisms in your gut.
Get a probiotic supplement
Get a prebiotic supplement (glyconutrients)
 FAO/WHO (2001) Health and Nutritional Properties of Probiotics in Food including Powder Milk with Live Lactic Acid Bacteria. Report of a Joint FAO/WHO Expert Consultation on Evaluation of Health and Nutritional Properties of Probiotics in Food Including Powder Milk with Live Lactic Acid Bacteria.
 WebMD.com. Ulcerative Colitis and Probiotics.
Available at: http://www.webmd.com/ibd-crohns-disease/ulcerative-colitis/features/ulcerative-colitis-and-probiotics.
Last accessed June 26, 2009
Sinnott RA, Ramberg J, Kirchner JM, Oubre C, Duncan C, Boyd S, Kalns JE (2007) Utilization of arabinogalactan, aloe vera gel polysaccharides, and a mixed saccharide dietary supplement by human colonic bacteria in vitro . Int J Probiotics Prebiotics 2, 97-104.