According to WHO, probiotics are "live microorganisms, which, when administered in adequate amounts, confer a health benefit on the host."
Many brands of yogurt claim to have live, active cultures. I believe the difference between them and the ones labeled probiotic is the
amount of active cultures in the product. Probiotic yogurt contains enough to have a health benefit rather than only an advertising benefit.
Check out the following link for more information.
USProbiotics.org