Probiotics are live microorganisms, usually bacteria but sometimes "friendly" yeasts, that provide us with health benefits when we ingest (eat) them.
Often, probiotic bacteria are the same types of "healthy" bacteria found in yogurt, including members of the Lactobacillus and/or Bifidobacteria bacterial families. These bacteria are normally found in our digestive tracts. In fact, when these bacteria are noticeably absent from our digestive system, diseases often occur.
Another widely studied probiotic is the yeast strain Saccharomyces boulardii. This yeast does not usually live inside our bodies. Instead, this yeast is usually found living on the lychee nut (a type of fruit). This yeast has been most widely studied for its role in treating chronic, persistent Clostridium difficile (C. diff) diarrhea.