Prebiotics are a category of non-digestible nutrients that stimulate the growth and activity of beneficial microflora in the colon.  They are especially known for boosting the important bifidobacteria and lactic acid bacteria.  

Typically, prebiotics are carbohydrates, and the most prevalent forms of prebiotics are nutritionally classed as soluble fiber. Prebiotics improve digestion and enhance nutrient absorption and the effectiveness and the immune system.

Traditional dietary sources of prebiotics include Jerusalem artichokes, jicama and chicory root--all which contain inulin.  Raw oats, unrefined wheat and unrefined barley also provide prebiotics.

Prebiotic oligosaccharides are increasingly added to foods for their health benefits. Some oligosaccharides that are used in this manner are fructooligosaccharides (FOS), xylooligosaccharides (XOS), polydextrose and galactooligosaccharides (GOS).

References

Gibson GR, Roberfroid MB. Dietary modulation of the human colonic microbiota: introducing the concept of prebiotics. J Nutr. 1995 Jun;125(6):1401-12. PMID