“a preparation (as a dietary supplement) containing live bacteria (as lactobacilli) that is taken orally to restore beneficial bacteria to the body ; also : a bacterium of such a preparation”

The Food and Agriculture Organization of the United Nations defines probiotic as

“Live microorganisms which when administered in adequate amounts confer a health benefit on the host.”

Our intestines are the host of hundreds of different types of bacteria, often referred to as micro flora or gut flora. Some of these bacteria are beneficial and some are not. Probiotics simply put are strains of the good bacteria. However there are many different types of probiotics. For example most often probiotics will come from one of two groups of bacteria, Lactobacillus or Bifdobacterium. However within each group there are different species and within each species there may be different strains.