Believe it or not, there are people who want you to eat live bacteria. Furthermore, there are other people in this world who want you to feed those bacteria you just ate so that they will grow and thrive.
The people who want you to eat bacteria are pro bacteria so they want you to eat food that contains probiotics. Note that antiseptics are sometimes called antibiotics-which literally means 'against life'. So probiotics are 'for life' or in favor of life.
Those who favor probiotics claim that they do everything from aiding in digestion to preventing disease. It seems certain that we need the right kind of bacteria in our gut to maintain good health. Whether or not a healthy person needs to be introducing more bacteria into his gut is in question.
Most of us have heard of acidophilus helping out people who are lactose intolerant and there is the well known belief that agents in mother's milk help confer immunity to disease to a nursing infant.
In the past decade research has been reported in Nature and by the NIH on the benefits conferred by probiotics for some illnesses that have eluded other means of treatment.
Prebiotics provide food for probiotics. In other words, prebiotics feed the bacteria in your gut. Some studies have found that once treatment with probiotics was stopped, they bacteria were eventually eliminated from the patient's body. Prebiotics may be a means for keeping the probiotics around a little longer.
Wherever you find people talking or writing about probiotics, prebiotics seems to want to tag along like a little sibling. However, probiotics appear to have a longer history and a greater weight of peer reviewed clinical evidence in their favor. Whether or not prebiotic supplements are a good idea seems to be an issue that is still unresolved.
One of the interesting side lights of prebiotics is that they are defined as indigestible fiber. What this can mean is that they are found in proteins and sugars that are hard to digest or which digest slowly. If you are trying to control sugar spikes in your blood for instance, it is possible that a form a glucose that is slow to be digested, a prebiotic, may be beneficial to you.
Since foods as varied as onions, bananas, and soybeans are thought to contain prebiotics, it may be that to the extent they are beneficial; a healthy, varied diet would already contain them.