Understanding the differences is useful, and quite important.
The word probiotic is being hijacked into other areas to mislead potential purchasers, strictly a probiotic is basically an organism (usually bacteria, sometimes a yeast) which can be given in food, or water or as a paste to animals (including humans) to have beneficial effects of the bacteria etc which currently live in that animals bowels. Some individuals for one reason or another have ‘disturbed’ bowels resulting in chronic pain, constipation, poor absorption, diarrhoea etc. This is often linked with an abnormal gut flora (the term for the bugs which normally live in the gut), sometimes this is caused by low grade infections with bacteria such as E.coli or Salmonella, or others from the host of bacterial species which can infect guts!
Prebiotics are natural, normally plant extracts which function as ‘soluble fibre’. Fibre assists the function of the gut in slowing the gut down – think of it as providing something for it to work on! Soluble fibre is a natural thickening agent, it helps to thicken gut contents and improve mixing and general gut movement, and in doing so encourages the growth and survival of probiotics which usually accompany it.
The term synbiotic has crept into use recently and usually means a product with both pre- and pro-biotics acting together synergistically (assisting each other) and actually producing a better effect than either on its own.
Over the last few years some companies have started to use the terms interchangeably for marketing purposes and to refer to products just containing a prebiotic as either a probiotic or a synbiotic. This could be viewed as misleading and may fall foul of the law now that these things are being defined in law.