Probiotics are defined as “live microorganisms that when consumed in sufficient amounts provide health benefits to the host.” The Food Agriculture Organization and World Health Organization. FAO/WHO, 2002.

First off, to understand why they would be helpful, you must know that all humans have natural gut microflora or bacteria that live on our skin, in our mouths and throughout our digestive tracts. It is important to keep a good balance between the good and bad bacteria in order to maintain health. Therefore, certain probiotics may be a cost-effective way to safely protect against infections.
Traditionally, probiotics are found in fermented foods such as yogurt and cultured milk but as interest has grown, they have been found as an added ingredient in other products (more added to certain yogurts, cottage cheese and milk, kefir, and cereals like Kashi Vive). The microorganisms only stay in the gut for a limited time unless you continue to eat the food. You can also take them in pill form. Speak to your doctor about these before starting.
Most people eat foods containing probiotics mostly for the digestive benefit. However, they also offer immunity, decrease the development of allergies in children, help keep bacteria in the gut when on antibiotics, and may assist in lactose intolerance.
Here is an excellent website for more information.
Emily Fonnesbeck RD,CD